Zero excess energy storage transformer

ABSTRACT

A drive circuit configured to drive a load. The drive circuit comprises a transformer. The transformer comprises a transformer primary comprising one or more primary windings connected to arms that pass through a magnetic material. The transformer also comprises a transformer secondary comprising secondary windings connected to planar secondary conductors which pass through the inside of the arms. The drive circuit also comprises a voltage source configured to apply a voltage across the transformer primary.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of and claims thebenefit of priority to U.S. Nonprovisional patent application Ser. No.16/325,972, filed Feb. 15, 2019, which is a 371 of internationalPCT/GB2017/052434, filed Aug. 17, 2017, which claims the benefit ofpriority to GB Patent Application No. 21614075.8, filed Aug. 17, 2016,the entirety of each of which is hereby incorporated by referenceherein.

TECHNICAL FIELD

The present disclosure relates generally to a transformer. Morespecifically, but not by way of limitation, this disclosure relates to atransformer having a reduced leakage inductance (inductance in onewinding that does not generate a magnetic field that induces a currentin the other winding).

BACKGROUND

There are many applications in which a capacitive load is driven so asto effect mechanical movement of a material. For example, in apiezo-electric actuator, capacitive piezo-electric crystals are drivenin order to cause mechanical movement of a material. The drive circuitsfor such applications are typically voltage driven. Charging and/ordischarging of the capacitive load of the drive circuit between desiredvoltage levels at a desired charge rate can be achieved with highprecision when the capacitive load has a known constant capacitance andthe drive signal is specifically designed to drive this. However, forapplications in which the capacitance of the load is not constant duringan operation involving several charging cycles, the drive signal is notwell matched to every one of the capacitive loads of the chargingcycles. A compromise drive signal is selected. Typically, when thecapacitive load is high, the compromise drive signal is unable to effectthe desired charging rate. Whilst, when the capacitive load is low, thecompromise drive signal causes significant overshooting of the maximumdesired voltage level, undershooting of the minimum desired voltagelevel and ringing of the voltage level when the capacitive load ischarged and discharged.

Thus, there is a need for improved drive electronics for drivingcapacitive loads and for improved transformers for driving circuitrywhich may include capacitive loads.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided circuitry fordriving a variable capacitive load comprising: a variable capacitiveload; a digital control circuit configured to generate a digital drivesignal; and a drive circuit configured to convert the digital drivesignal into an analogue drive signal, the analogue drive signal forminga drive waveform for charging the variable capacitive load, the drivecircuit comprising a slewing circuit configured to drive a timedependent voltage component of the drive waveform; wherein the digitalcontrol circuit is configured to modify the digital drive signal foreach charging cycle so as to match the time dependent voltage componentof the drive waveform to the variable capacitive load.

The digital control circuit may be configured to modify the digitaldrive signal so as to cause the time dependent voltage component of thedrive waveform to maintain a constant slew rate during charging of thevariable capacitive load.

The analogue drive signal may further form a drive waveform fordischarging the variable capacitive load, and the digital controlcircuit may be configured to modify the digital drive signal so as tocause the time dependent voltage component of the drive waveform tomaintain a constant slew rate during discharging of the variablecapacitive load.

The drive circuit may further comprise a keeper circuit coupled to theslewing circuit, the keeper circuit configured to drive a static voltagecomponent of the drive waveform.

The slewing circuit may comprise a current source, and the digitalcontrol circuit may be configured to modify the digital drive signal soas to change the amplitude of the slewing circuit current driving thetime dependent voltage component of the drive waveform.

The slewing circuit may comprise a current source, and the digitalcontrol circuit may be configured to modify the digital drive signal soas to change the period of the slewing circuit current driving the timedependent voltage component of the drive waveform.

The slewing circuit may comprise a voltage source, and the digitalcontrol circuit may be configured to modify the digital drive signal soas distort the analogue drive signal.

After discharging and during charging of the variable capacitive load,the digital control circuit may be configured to generate the digitaldrive signal so as to cause the keeper circuit to maintain a staticvoltage component of the drive waveform at a desired pre-charge value.

For each charging cycle, the digital control circuit may be configuredto modify the desired pre-charge value so as to match the variablecapacitive load.

After charging and during discharging of the variable capacitive load,the digital control circuit may be configured to generate the digitaldrive signal so as to cause the keeper circuit to maintain a staticvoltage component of the drive waveform at a desired pre-dischargevalue.

For each charging cycle, the digital control circuit may be configuredto modify the desired pre-discharge value so as to match the variablecapacitive load.

The digital control circuit may be configured to modify the digitaldrive signal for each charging cycle so as to match the time dependentvoltage component of the drive waveform to a determined capacitance ofthe variable capacitive load for that charging cycle, wherein thecapacitance of the variable capacitive load for that charging cycle ispredetermined.

The predetermined capacitance of the variable capacitive load may betemperature dependent.

At any time the variable capacitive load may comprise an active subsetof a set of capacitive loads, wherein the capacitance of each capacitiveload is predetermined, and for each charging cycle, the digital controlcircuitry may be configured to: receive an indication of the activesubset of capacitive loads for that charging cycle; and determine thevariable capacitive load for that charging cycle based on the activesubset of capacitive loads for that charging cycle and the predeterminedcapacitance of each capacitive load.

The predetermined capacitance of each capacitive load may be temperaturedependent.

The predetermined capacitance of each capacitive load of the activesubset may be the same.

At any time the variable capacitive load may comprises an active subsetof a set of capacitive loads, and the digital control circuit may beconfigured to determine the capacitance of each capacitive load by:controlling that capacitive load only to be connected to the drivecircuit; generating a test digital drive signal for driving thatcapacitive load only, the test drive signal for being converted by thedrive circuit to a test analogue drive signal forming a test drivewaveform for charging that capacitive load only; measuring the timedependent voltage component of the test drive waveform of thatcapacitive load; and deriving the capacitance of that capacitive loadfrom the measured time dependent voltage component of the test drivewaveform of that capacitive load.

The digital control circuit may be configured to measure the timedependent voltage component of the test drive waveform of thatcapacitive load by measuring the time dependent voltage component of thetest analogue drive signal of that capacitive load.

The digital control circuit may be further configured to for eachcapacitive load, compare the maximum voltage of the measured timedependent voltage component of the test drive waveform of thatcapacitive load to a desired post-charge voltage.

The digital control circuit may be further configured to for eachcapacitive load, compare the minimum voltage of the measured timedependent voltage component of the test drive waveform of thatcapacitive load to a desired post-discharge voltage.

The digital control circuit may be further configured to determine thetemperature dependence of the capacitance of each capacitive load byrepeating the steps of: controlling that capacitive load only to beconnected to the drive circuit; generating a test digital drive signalfor driving that capacitive load only, the test drive signal for beingconverted by the drive circuit to a test analogue drive signal forming atest drive waveform for charging that capacitive load only; measuringthe time dependent voltage component of the test drive waveform of thatcapacitive load; and deriving the capacitance of that capacitive loadfrom the measured time dependent voltage component of the test drivewaveform of that capacitive load, when the variable capacitive load haseach of a set of temperatures.

The digital control circuit may be further configured to determine thetemporal dependence of the capacitance of each capacitive load byrepeating the steps of: controlling that capacitive load only to beconnected to the drive circuit; generating a test digital drive signalfor driving that capacitive load only, the test drive signal for beingconverted by the drive circuit to a test analogue drive signal forming atest drive waveform for charging that capacitive load only; measuringthe time dependent voltage component of the test drive waveform of thatcapacitive load; and deriving the capacitance of that capacitive loadfrom the measured time dependent voltage component of the test drivewaveform of that capacitive load, when the variable capacitive load hasbeen active for each of a set of time periods.

The digital control circuit may be configured to determine thecapacitance of each capacitive load during manufacture so as to generatea predetermined capacitance of each capacitive load.

The digital control circuit may be configured to determine thecapacitance of each capacitive load prior to an operational use of thevariable capacitive so as to generate a predetermined capacitance ofeach capacitive load.

The digital control circuit may be configured to determine thecapacitance of each capacitive load between charging cycles of thevariable capacitive load.

The circuitry may further comprise a comparator configured to receive anindication of the drive waveform voltage and a threshold voltage, andchange state when the drive waveform voltage crosses the thresholdvoltage.

The digital control circuit may be configured to halt outputting thedigital drive signal to the drive circuit in response to the comparatorchanging state.

The indication of the drive waveform voltage may be the drive waveformvoltage.

The indication of the drive waveform voltage may be the analogue drivesignal voltage.

The digital control circuit may be configured to generate the thresholdvoltage, and the digital control circuit may be configured to modify thethreshold voltage for each charging cycle so as to cause the maximumdrive waveform voltage to match a desired post-charge voltage.

The digital control circuit may be configured to generate the thresholdvoltage, and the digital control circuit may be configured to modify thethreshold voltage for each charging cycle so as to cause the minimumdrive waveform voltage to match a desired post-discharge voltage.

The digital control circuit may be configured to detect the elapsed timein each charging cycle at which the comparator changes state.

The digital control circuit may be configured to detect the temperatureof the variable capacitive load in each charging cycle at which thecomparator changes state.

The digital control circuit may be configured to detect a fault in thevariable capacitive load by adjusting the detected elapsed time for thedetected temperature to form an adjusted detected elapsed time, andcompare the adjusted detected elapsed time to at least one thresholdtime.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example withreference to the accompanying drawings. In the drawings:

FIG. 1 illustrates drive circuitry of the invention;

FIG. 2 illustrates a prior art inkjet actuator drive circuit;

FIGS. 3 a and 3 b illustrate plots for charging cycles of a prior artcapacitive load driving circuit;

FIG. 4 illustrates a current drive circuit for driving printheadactuators;

FIG. 5 illustrates a further current drive circuit for driving printheadactuators;

FIG. 6 illustrates a voltage drive circuit for driving printheadactuators;

FIGS. 7 a and 7 b illustrate plots for charging cycles of a currentdriven printhead actuator circuit of the type shown in FIGS. 4 and 5 ;

FIG. 8 illustrates a plot for a charging cycle of a voltage drivenprinthead actuator circuit of the type shown in FIG. 6 ;

FIG. 9 illustrates an enlarged view of the amplifier drive waveform ofFIG. 8 ;

FIG. 10 illustrates the threshold voltages of a comparator used todisable the drive signal of the drive circuit;

FIG. 11 illustrates a known EDM arrangement;

FIG. 12 illustrates an EDM drive circuit;

FIGS. 13 a, 13 b and 13 c illustrate plots for charging cycles of theEDM drive circuit;

FIGS. 14 a, 14 b and 14 c illustrate variations in spark gapcapacitances due to workpiece geometry;

FIGS. 15 a, 15 b, 15 c, 15 d, 15 e, 15 f and 15 g illustrate variationsin spark gap capacitances due to workpiece geometry;

FIGS. 16 a and 16 b illustrate the surface finish of a workpiecemachined using the EDM drive circuit of FIG. 12 ;

FIGS. 17 a and 17 b illustrate a transformer for use in an EDM drivecircuit;

FIGS. 18, 19 and 20 illustrate further EDM drive circuits;

FIG. 21 illustrates a 3D metal printer architecture;

FIG. 22 illustrates a fusion pulse power supply drive circuit;

FIGS. 23 a, 23 b and 23 c illustrate plots for charging cycles of thefusion pulse power supply drive circuit;

FIGS. 24 a, 24 b and 24 c illustrate example drive circuits comprisingthe transformer of FIGS. 17 a and 17 b;

FIGS. 25 a, 25 b, 26 a and 26 b illustrate examples of transformer crosssections; and

FIG. 27 illustrates an example of a transformer topology.

DETAILED DESCRIPTION

The following describes circuitry for driving variable capacitive loadsin which the signal which drives the charging (and optionallydischarging) of the variable capacitive load is dynamically modified soas to match the current capacitance of the variable capacitive load.

The block diagram of FIG. 1 illustrates the principle components of thedrive circuitry 100. Digital control circuit 102 generates and outputs adigital drive signal 104. Digital drive signal 104 is input to drivecircuit 106 to control the operation of drive circuit 106. Drive circuit106 converts the digital drive signal 104 into an analogue drive signal108. Analogue drive signal 108 is output from drive circuit 108.Analogue drive signal 108 forms drive waveform 110 which drives thecharging of the variable capacitive load 112. The analogue drive signal108 morphs into the drive waveform 110 as it passes from the drivecircuit 106 to the variable capacitive load 112.

The drive circuit comprises a slewing circuit which drives a timedependent voltage component of the analogue drive signal, and hencedrives a time dependent voltage component of the drive waveform. Thechanging voltage over time of the time dependent voltage component ofthe drive waveform drives the charging of the variable capacitive load.

In order to precisely control the operation of the variable capacitiveload, the rate of change of voltage over time of the drive waveform (theslew rate), the minimum voltage of the drive waveform (Vlow), and themaximum voltage of the drive waveform (Vhigh) are precisely controlledand maintained at their desired values as the capacitance of thevariable capacitive load varies. In particular, the digital controlcircuit modifies the digital drive signal for each charging cycle so asto maintain a constant slew rate of the time dependent voltage componentof the drive waveform during charging of the variable capacitive load.The digital control circuit may also modify the digital drive signal foreach charging cycle so as to maintain a constant slew rate of the timedependent voltage component of the drive waveform during discharging ofthe variable capacitive load.

Circuitry 100 may be current driven or voltage driven. The specificcapacitive load conditions of each charging cycle may be known inadvance, or may be measured on the fly for each charging cycle or set ofcharging cycles in a feedback loop to the digital control circuit.

Circuitry 100 is applicable for use in driving variable capacitive loadsin many technology areas. The following describes exemplary uses ofcircuitry 100 in printhead actuator driving, electro discharge machiningand 3D metal printing.

In the following example, the circuitry of FIG. 1 is used to driveprinthead actuators. The actuators may be one or both of piezoelectricand thin film MEMs actuators. Each printhead actuator is a capacitiveload. Driving the capacitive load actuates the printhead nozzle andcauses ink to be propelled from the nozzle onto a surface. The printercomprises a set of printhead actuators. The printer prints in printcycles. Each print cycle typically corresponds to a line of print. Onlya subset of the printhead actuators is used on each print cycle to ejectink onto the surface. Thus, the active subset of printhead actuators isa variable capacitive load which varies by print cycle. The combinedcapacitance of the active subset of printhead actuators is thecapacitance being driven by the drive circuitry on that print cycle.

FIG. 2 illustrates a known inkjet actuator drive circuit 200. The bankof printhead actuators is represented by variable capacitive load 222.Digital control circuitry 202 generates and outputs a voltage digitaldrive signal 204. Drive circuit 206 receives the voltage digital drivesignal 204 and converts it to an analogue drive signal 208. Drivecircuit 206 comprises a digital to analogue converter (DAC) 224, avoltage amplifier 226 and a resistor 228. The analogue drive signalcomprises a time dependent voltage component. The analogue drive signal208 propagates along cable 230 to form the drive waveform at the bank ofprinthead actuators. Cold switch application specific integrated circuit(ASIC) 232 switches the drive waveform through to the active subset ofprinthead actuators which are scheduled to eject ink in that printcycle. The cold switch ASIC receives a control signal 234 from digitalcontrol circuitry 202 which identifies which printhead actuators areactive for that print cycle. The return current flows along the cable230 to ground return 236. The time dependent voltage component of thedrive waveform causes the active subset of printhead actuators to ejectink in each print cycle.

The voltage amplifier 226 is set up to drive both the minimumcapacitance of the variable capacitive load when none of the printheadnozzles is ejecting ink on a print cycle, and the maximum capacitance ofthe variable capacitive load when all of the printhead nozzles areejecting ink on a print cycle. The capacitance of the variablecapacitive load may therefore vary between 0 nF and a few hundred nF.For example, the capacitance of the variable capacitive load may varybetween 0 nF and 300 nF. When the capacitance of the variable capacitiveload changes, this affects the slew rate of the drive waveform, themaximum and minimum voltages of the drive waveform, and the uniformityof the drive waveform.

FIGS. 3 a and 3 b illustrate plots for an inkjet actuator drive circuitof the type shown in FIG. 2 . In each figure, plot A is the drive signalinput to the voltage amplifier 226, plot B is the analogue drive signaloutput from the drive circuit, plot C is the current of the drivewaveform input to the variable capacitive load, and plot D is thevoltage across the variable capacitive load. In FIGS. 3 a and 3 b , theinkjet actuator drive circuit is optimised to drive a capacitance of 1nF. The capacitive load on FIG. 3 a is 1 nF. The capacitive load on FIG.3 b is 40 nF. In FIG. 3 a , the analogue voltage drive signal outputfrom the drive circuit is coincident with the voltage across thevariable capacitive load. This is because the inkjet actuator drivecircuit has been optimised to drive the load of 1 nF. In FIG. 3 b , theanalogue drive signal output from the drive circuit is not coincidentwith the voltage across the variable capacitive load. The slew rate ofplot D is not constant, the minimum voltage of plot D undershoots theintended minimum voltage of 0V, the maximum voltage of plot D overshootsthe intended maximum voltage of 22V, and there is ringing of the voltagesignal between the charging and discharging portions of plot D. Thus,the inkjet actuator drive circuit does not drive the capacitive load of40 nF optimally. These factors lead to the printhead actuators notejecting the ink as intended, and hence cause the quality of theresultant printed image being degraded. Specifically, in the system ofFIG. 2 , the amount of ink ejected from each nozzle is dependent on thenumber of nozzles actuated in that print cycle. Thus, the volume andvelocity of the ejected ink droplets is data-dependent. The lifetimes ofthe actuators are also negatively affected by the mismatch between theinkjet actuator drive circuit's optimal capacitive load and its actualcapacitive load on a print cycle. Specifically, the actuators arestressed by the signal ringing shown on FIG. 3 b . Frequent exposure tothis signal ringing reduces their mean time between failure (MTBF).

It is known to statically optimize the inkjet actuator drive circuit todrive the entire range of capacitive loads with the best averageperformance. The inkjet actuator drive circuit may be nominallyoptimised for the minimum, maximum or another capacitive load in thefull range of capacitive loads, and compensated to ensure that theundershoot, overshoot and ringing issues shown in FIG. 3 b areacceptable across the capacitive load range. However, this leads to adrive signal which is not well matched to every capacitive load of therange of variable capacitive loads, and hence to sub-optimal drivingacross the capacitive load range.

The drive waveform is distorted relative to the analogue drive signal inpart due to considerable inductance caused by the length of the cable230. It is known to minimise the length of the cable to reduce theinductance.

It is also known to perform data-dependent trimming of the controlsignal 234 at the digital control circuitry 202 in an effort tocompensate for the data-dependency of the ink ejected from the actuatornozzles. Specifically, the number and size of the ink droplets that makeup a full printed pixel may be modified on the basis of an algorithmthat takes the number of nozzles printed on the current and previousrows into account. These digital domain trims require additionalprocessing power and on-chip area, and do not remove all data dependentartefacts, and hence are an incomplete and undesirable solution.

FIG. 4 illustrates a first exemplary current drive circuit 400 fordriving printhead actuators. The bank of printhead actuators isrepresented by variable capacitive load 422. Each printhead actuator mayhave a capacitance in the range of 200 to 1 nF. The bank of printheadactuators may comprise several hundred individual printhead actuators.In a typical print cycle, between 100 and 250 of the printhead actuatorsare actuated. If no pixels are to be printed in a print cycle, then noprinthead actuators eject ink, and hence the capacitance of the variablecapacitive load is 0 pf. If in a print cycle all the printhead actuatorseject ink, then the capacitance of the variable capacitive load may be afew hundred nF.

Digital control circuitry 402 generates and outputs a current digitaldrive signal 404. Drive circuit 406 receives the current digital drivesignal 404 and converts it to an analogue drive signal 408. The analoguedrive signal 408 propagates along cable 430 to form the drive waveformat the bank of printhead actuators. Cable 430 may be a flexible printedcircuit (FPC) or a flexible printed circuit board (PCB) cable. Cable 430may be between 100 and 500 mm long and have parameters in the followingranges: 30-150 nH, 10-50 pF, 0.05-0.25 Ohms, 0.5-2.5 ns Tpd.

Cold switch application specific integrated circuit (ASIC) 432 switchesthe drive waveform through from the drive circuit 406 to the activesubset of printhead actuators which are scheduled to eject ink in thatprint cycle. The cold switch ASIC receives a control signal 434 fromdigital control circuitry 402 which identifies which printhead actuatorsare active for that print cycle. For example, the control signal 434 maycomprise pixel data. The control signal 434 may identify the pixel datafor each print row. The cold switch ASIC 432 connects the drive waveform410 through to the subset of printhead actuators indicated by the pixeldata for that print cycle. The time dependent voltage component of thedrive waveform causes the active subset of printhead actuators to ejectink in each print cycle. Capacitor 438 is connected in parallel withvariable capacitive load 422. Capacitor 438 is separated from variablecapacitive load 422 by cold switch ASIC 432. Capacitor 438 may have acapacitance of ^(˜)1 nF. The return current flows along the cable 430 toground return 436.

Drive circuit 406 comprises a keeper circuit coupled to a slewingcircuit. The keeper circuit comprises a digital to analogue converter(DAC) 424, a voltage amplifier 426 and a resistor 428. The currentdigital drive signal 404 is input to the DAC 424. The output of the DAC424 is input to voltage amplifier 426. The output of voltage amplifier426 is input to resistor 428. The keeper circuit drives a static voltagecomponent of the analogue drive signal. Hence, the keeper circuit drivesa static voltage component of the drive waveform. Circuit 400 achievesthis by the digital drive signal 404 driving DAC 424 at all times.Resistor 428 may have a resistance in the range of 100-1000 Ohms. Theoutput of the voltage amplifier 426 is driven through this resistor toprovide a low drive strength static voltage component to the analoguedrive signal. After discharging the actuators and during charging of theactuators, the digital control circuit 402 generates the digital drivesignal 404 so as to cause the keeper circuit to maintain the staticvoltage component of the drive waveform at a desired pre-charge value,Vlow. Vlow may be in the range 0-2V. After charging the actuators andduring discharging of the actuators, the digital control circuit 402generates the digital drive signal 404 so as to cause the keeper circuitto maintain the static voltage component of the drive waveform at adesired pre-discharge value, Vhigh. Vhigh may be in the range 20-40V.The keeper circuit compensates for leakage current through cold switchASIC 432 and actuator bank 422 which would otherwise pull the voltage ofthe drive waveform 410 down. The keeper circuit maintains the DC levelof the drive waveform 410 at the desired one of Vhigh and Vlow despitethe leakage current.

The slewing circuit comprises DACs 440 and 446, each of which receivesas an input the digital drive signal 404. The output of DAC 440 drivescurrent source 442. Current source 442 is connected to high voltage rail444. The high voltage rail 444 may be between 30 and 40V. The output ofDAC 446 drives current source 448. Current source 448 is connected tolow voltage rail 450. The low voltage rail 450 may be between −5 and−10V. The outputs of both current sources are connected to the output ofresistor 428. The slewing circuit slews the voltage of the analoguedrive signal from one voltage to another. Positive current from currentsource 442 ramps the voltage Vlow of the signal output from the keepercircuit up to Vhigh during charging of the printhead actuator bank.Negative current from current source 448 ramps the voltage Vhigh of thesignal output from the keeper circuit down to Vlow during discharging ofthe printhead actuator bank. The slewing circuit drives the timedependent voltage component of the analogue drive signal. Hence, theslewing circuit drives the time dependent voltage component of the drivewaveform. The slewing circuit does not provide current to the analoguedrive signal at any other time. Thus, the drive circuit comprises thekeeper circuit to maintain the DC level of the analogue drive signal atthe desired level.

The digital drive signal 404 controls the characteristics of thecharging current from current source 442 applied to the signal outputfrom the keeper circuit. The digital drive signal 404 controls thecharacteristics of the discharging current from current source 448applied to the signal output from the keeper circuit. The digitalcontrol circuit 402 may modify the digital drive signal 404 so as tochange the amplitude and/or period of the charging current to match thepositive voltage slew rate of the drive waveform to the variablecapacitive load of the current charging cycle. The digital controlcircuit 402 may modify the digital drive signal 404 so as to change theamplitude and/or period of the discharging current to match the negativevoltage slew rate of the drive waveform to the variable capacitive loadof the current charging cycle.

FIG. 5 illustrates a second exemplary current drive circuit 500 fordriving printhead actuators. The current drive system of FIG. 5 is thesame as that of FIG. 4 except that it has a different arrangement forthe slewing circuit of the drive circuit 506. Those components in commonwith those of FIG. 4 are as described with reference to FIG. 4 .

The slewing circuit of FIG. 5 comprises a DAC 552 which receives as aninput the digital drive signal 504. The output of DAC 552 drives currentsource 558. High voltage rail 544 is connected to current source 558 viaswitch 554. The high voltage rail 544 may be between 30 and 40V. Lowvoltage rail 550 is connected to current source 558 via switch 556. Thelow voltage rail 550 may be between −5 and −10V. The output of currentsource 558 is connected to the output of resistor 528. One way diodes560 a,b,c,d surround current source 558, such that when switch 544 isclosed and switch 556 open, the high voltage rail 544 connects to thecurrent source through diode 560 a which drives a charging currentthrough diode 560 c to the output of the keeper circuit. Thus, positivecurrent from current source 558 ramps the voltage Vlow of the signaloutput from the keeper circuit up to Vhigh during charging of theprinthead actuator bank. Meanwhile, when switch 556 is closed and switch544 open, the low voltage rail 550 connects to the current sourcethrough diode 560 d. The current source is connected to the output ofthe keeper circuit through diode 560 b. Thus, negative current fromcurrent source 558 ramps the voltage Vhigh of the signal output from thekeeper circuit down to Vlow during discharging of the printhead actuatorbank. In this way, the slewing circuit slews the voltage of the analoguedrive signal from one voltage to another. The slewing circuit drives thetime dependent voltage component of the analogue drive signal. Hence,the slewing circuit drives the time dependent voltage component of thedrive waveform. The slewing circuit does not provide current to theanalogue drive signal at any other time. Thus, the drive circuitcomprises the keeper circuit to maintain the DC level of the analoguedrive signal at the desired level. The digital drive signal 504 controlsthe characteristics of the charging current and or/discharging currentof FIG. 5 as described with reference to FIG. 4 .

FIGS. 4 and 5 illustrate two exemplary current drive circuitarrangements for driving a variable capacitive load. However, it will beunderstood that any suitable slewing circuit arrangement may beimplemented to slew the analogue drive signal between the DC voltagelevels provided by the keeper circuit. For example, a high speed digitalpulse width modulation (PWM) unit with an RF GaN pass transistor may beimplemented instead of the analogue current sources shown in FIGS. 4 and5 .

FIG. 6 illustrates an exemplary voltage drive circuit 600 for drivingprinthead actuators. The voltage drive system of FIG. 6 is the same asthat of FIGS. 4 and 5 except that it is voltage driven as opposed tocurrent driven and has a different arrangement for the drive circuit606. Those components in common with those of FIG. 4 are as describedwith reference to FIG. 4 .

Digital control circuitry 602 generates and outputs a voltage digitaldrive signal 604. Drive circuit 606 receives the voltage digital drivesignal 604 and converts it to an analogue drive signal 608. The analoguedrive signal 608 converts to the drive waveform 610 and drives theactuator bank 622 under the control of digital control circuit 602 asdescribed with reference to FIG. 4 .

Drive circuit 606 comprises a digital to analogue converter (DAC) 624and a voltage amplifier 626. The voltage digital drive signal 604 isinput to the DAC 624. The output of the DAC 624 is input to voltageamplifier 626.

The voltage amplifier 626 is connected to high voltage rail 644 and lowvoltage rail 650. High voltage rail 644 may be between 30 and 40V. Lowvoltage rail 650 may be between −5 and −10V. The voltage amplifiergenerates an output having a voltage between the voltage of the high andlow voltage rails. In accordance with the control signal from digitalcontrol circuit 602, the voltage output from the voltage amplifier slewsfrom one voltage to another. The output of the voltage amplifier is theanalogue drive signal. Positive voltage from voltage rail 644 ramps thevoltage of the voltage amplifier output up to Vhigh during charging ofthe printhead actuator bank. Negative voltage from voltage rail 650ramps the voltage of the voltage amplifier output down to Vlow duringdischarging of the printhead actuator bank.

The digital drive signal 604 controls the characteristics of thecharging voltage from high voltage rail 644 applied to the voltageamplifier 626. The digital drive signal 604 controls the characteristicsof the discharging voltage from low voltage rail 650 applied to thevoltage amplifier 626. The digital control circuit 602 may modify thedigital drive signal 604 so as to distort the analogue drive signal 608such that the positive voltage slew rate of the drive waveform 610 ismatched to the variable capacitive load of the current charging cycle.The digital control circuit 602 may modify the digital drive signal 604so as to distort the analogue drive signal 608 such that the negativevoltage slew rate of the drive waveform 610 is matched to the variablecapacitive load of the current charging cycle.

FIG. 6 illustrates an exemplary voltage drive circuit arrangement fordriving a variable capacitive load. However, it will be understood thatany suitable drive circuit may be used to pre-distort the analogue drivesignal to form a drive waveform 610 that is matched to the variablecapacitive load of the current charging cycle.

As described above, whether the drive circuit for the printheadactuators is current driven as shown in FIGS. 4 and 5 , or voltagedriven as shown in FIG. 6 , the digital control circuit dynamicallymodifies the digital drive signal for each printing cycle so as to matchthe time dependent voltage component of the drive waveform to thecapacitance of the printhead actuators in that printing cycle.

FIGS. 7 a and 7 b illustrate plots for a current driven printheadactuator circuit of the type shown in FIGS. 4 and 5 . In each figure,plot A is the current of the drive waveform input to the variablecapacitive load, and plot B is the voltage across the variablecapacitive load. The capacitive load on FIG. 7 a is 40 nF. Thecapacitive load on FIG. 7 b is 1 nF. For FIG. 7 a , the rate of changeof voltage with time is constant over the transition from Vhigh to Vlow.The rate of change of voltage with time is also constant over thetransition from Vlow to Vhigh. There is no roll off at the ends of thetransitions of the type visible in FIGS. 3 a and 3 b . There is noovershoot above the desired Vhigh and no undershoot below the desiredVlow. The voltage signal does not exhibit any ringing. For FIG. 7 b ,the transitions between the voltage levels are also highly linear. Theslew rate is the same for the 1 nF load as it is for the 40 nF load.There is no overshoot above the desired Vhigh and no undershoot belowthe desired Vlow. The voltage signal does not exhibit any ringing. Thereis a small roll off at the start of the transition from Vhigh to Vlow asthe turn off time of the current source is slower at very low currents.The digital control circuit could peak the digital drive signal to forma peak in the analogue drive signal in order to prevent the identifiedroll off. In both cases, the current of the drive waveform input to thevariable capacitive load is a square wave. A negative square wave causesthe voltage transition from Vhigh to Vlow. A positive square wave causesthe voltage transition from Vlow to Vhigh. The digital control circuitdynamically modifies the digital current drive signal to match thevariable capacitive load of the print cycle by changing the amplitude ofthe square wave and/or the time period for which the current is at itsmaximum amplitude. In this way, the printhead actuators are driven witha drive waveform which is optimised for the capacitive load of thatprint cycle.

FIG. 8 illustrates a plot for a voltage driven printhead actuatorcircuit of the type shown in FIG. 6 . Plot A is the drive signal inputto the voltage amplifier 626, plot B is the analogue drive signal outputfrom the drive circuit, plot C is the current of the drive waveforminput to the variable capacitive load, and plot D is the voltage acrossthe variable capacitive load. The capacitive load on FIG. 8 is 40 nF. Bycomparison with FIG. 3 b , it can be seen that the drive signal input tothe amplifier drive waveform has been distorted. FIG. 9 illustrates anenlarged view of the distorted amplifier drive waveform. The amplifierdrive waveform has been distorted to optimise it to prevent overshootingof Vhigh and undershooting of Vlow. The voltage across the capacitiveload shown in FIG. 8 on plot D does not exhibit the overshooting andundershooting which can be seen in plot D of FIG. 3 b . Furtherdistortion could be applied to optimise the linearity of the voltagetransitions of pot D of FIG. 8 .

In both the current driven and voltage driven circuits, the digitalcontrol circuit receives or generates pixel data for the printingcycles. Thus, the number of printhead actuators active in each printcycle is known by the digital control circuit. The digital controlcircuit provides the pixel data to the cold switch ASIC. The digitalcontrol circuit modifies the digital drive signal dependent on thedetermined capacitance of the active printhead actuators for that printcycle.

The capacitance of the active printhead actuators for that print cyclemay be predetermined. For example, the digital control circuit mayreceive an indication of the predetermined capacitance of the variablecapacitive load for that printing cycle. Alternatively, the digitalcontrol circuit may store capacitance values for the printheadactuators, and determine the capacitance for a printing cycle based onthe number of active printhead actuators and the stored capacitancevalues. The digital control circuit may store only one nominalcapacitance for a printhead actuator, and determine the capacitance in aprinting cycle to be the nominal capacitance multiplied by the number ofprinthead actuators active in the printing cycle. Alternatively, thedigital control circuit may store a nominal capacitance for eachindividual printhead actuator, or for a group of printhead actuators,and determine the capacitance in a printing cycle to be the summation ofthe nominal capacitances of the printhead actuators active in theprinting cycle. The nominal capacitances may have been determined fromproduction sampling or design analysis.

The capacitance of a capacitive load is temperature dependent. Atemperature sensor (not shown) may measure the temperature of theprinthead actuators, and feed this temperature measurement back to thedigital control circuit. The digital control circuit may then modify thepredetermined capacitance of the active printhead actuators in theprinting cycle according to a stored capacitance-temperaturerelationship. Alternatively, the digital control circuit may store a setof values for the nominal capacitance or nominal capacitances of theprinthead actuators, each value for a different temperature range. Inthis case, the digital control circuit selects the nominal capacitancesof the set which correspond to the measured temperature.

The capacitance of each printhead actuator or groups of printheadactuators may be determined by direct measurement before the printingoperation is initiated. In this case, the digital control circuit storesthe measured capacitances, and then determines the capacitance in aprinting cycle to be the summation of the measured capacitances of theactive printhead actuators in the printing cycle. A temperature sensormay measure the temperature of the printhead actuators, and feed thistemperature measurement back to the digital control circuit. The digitalcontrol circuit may then modify the capacitance of the active printheadactuators in the printing cycle according to a storedcapacitance-temperature relationship. Alternatively, the digital controlcircuit may, before the printing operation is initiated, measure thecapacitance of each printhead actuator or groups of printhead actuatorsin different temperature ranges, and store a set of values for thecapacitances of the printhead actuators, each value for a differenttemperature range. In this case, the digital control circuit selects thecapacitances of the set which correspond to the measured temperature.

The capacitance of each printhead actuator or group of printheadactuators may be determined by direct measurement during the printingoperation in between active printing cycles. In this case, the digitalcontrol circuit stores the most recent measured capacitances, and thendetermines the capacitance in a printing cycle to be the summation ofthe measured capacitances of the active printhead actuators in theprinting cycle. No temperature adjustment is necessary in this case,because the temperature of the active printhead actuators of a printingcycle can be assumed to be the same as the temperature of thoseprinthead actuators when their capacitance was measured.

$\begin{matrix}{{I = {C\frac{dv}{dt}}}\;} & \left( {{equation}\mspace{14mu} 1} \right)\end{matrix}$

Where I is current, C is capacitance, V is voltage, and t is time.

For a current drive circuit, for each printing cycle, the digitalcontrol circuit modifies the current to match the desired rate of changeof voltage dv/dt to the determined capacitance for that printing cycle.The current of the drive waveform has a square wave profile, such asthat shown in FIGS. 7 a and 7 b . The current applied by the digitalcontrol circuit is directly proportional to the determined capacitanceof the variable capacitive load for that printing cycle. If theprinthead actuators are treated as each having the same capacitance,then the digital control circuit scales the amplitude of the current ofthe drive waveform of a printing cycle proportionally to the number ofactive printhead actuators in that printing cycle. Otherwise, thedigital control circuit scales the amplitude of the current of the drivewaveform of a printing cycle proportionally to the determinedcapacitance of the active printhead actuators in that printing cycle.The digital control circuit may also match the width of the currentpulses for the printing cycle to the determined capacitance for thatprinting cycle. The lower the capacitance, the smaller the width of thecurrent pulse. This is to avoid overshooting of Vhigh and undershootingof Vlow. The digital control circuit may also match Vhigh and Vlow tothe determined capacitance for that printing cycle.

For a voltage drive circuit, for each printing cycle, the digitalcontrol circuit modifies the digital voltage drive signal to distort theanalogue drive signal so as to cause the drive waveform to match thedesired rate of change of voltage dv/dt to the determined capacitancefor that printing cycle. The digital control circuit may store a set ofmodified digital voltage drive signals, each for driving a differentrange of capacitive loads. The digital control circuit selects thestored modified digital voltage drive signal corresponding to thedetermined capacitance of the current printing cycle, and applies thatselected modified digital voltage drive signal to the drive circuit.

The capacitance of an individual printhead actuator or a group ofprinthead actuators may be determined as follows. The digital controlcircuitry sends a control signal to cold switch ASIC. This controlsignal controls cold switch ASIC to connect the drive waveform to thecapacitive load to be measured. The control signal may, for example,cause a FET pass gate in the cold switch ASIC to be turned on whichconnects the drive waveform to the capacitive load. The capacitive loadmay be an individual printhead actuator or a group of printheadactuators.

The digital control circuit then generates a test digital drive signalfor driving that capacitive load only. The test digital drive signal maybe of the same form as a digital drive signal used during printing. Thetest digital drive signal may have a lower amplitude pulse and/or alower dv/dt pulse so as to enable the capacitance of the printheadactuator(s) to be determined without causing them to eject ink. Thedrive circuit converts the test digital drive signal to a test analoguedrive signal for charging the capacitive load. The test analogue drivesignal morphs to a test drive waveform for charging the capacitive load.

The test drive waveform is sensed and fed back to the digital controlcircuitry. The sensing may be near end sensing, in which the testanalogue drive signal and the return waveform are both sensed at theoutput of the drive circuit 470,570,670 and 472,572,672. The sensing mayalternatively, or additionally, be far end sensing, in which the testdrive waveform and the return waveform are both sensed 474,574,674 and476,576,676 at the printhead actuator bank end of cable 430,530,630.

The sensed signals are digitised. The sensed signals may be digitisedusing a high speed ADC. FIGS. 4, 5 and 6 illustrate an alternativeapproach, in which the sensed signals are amplified and repetitivelyinput to a low speed ADC 492,592,692 which receives a clock signal494,594,694 from digital control circuit 402,502,602. The clock signalis skewed in steps for each waveform pulse generated. For example, theclock signal may be skewed in 100 ps steps. This gives the equivalenttime sampling of 10 GHz for the ADC. Thus, for example, for a 2 mspulse, with a pulse repetition rate of 100 KHz, it would take 20 ms todigitise the entire waveform. The digitised signal is output from theADC to the digital control circuit.

In the case that both far end sensing and near end sensing areperformed, the far end sensed signals may be input 478,578,678 and480,580,680 to differential amplifier 486,586,686, and the near endsensed signals may be input 482,582,682 and 484,584,684 to differentialamplifier 488,588,688. The outputs of the two differential amplifiersare input to analogue multiplexor 490,590,690, whose output forms theinput to ADC 492,592,692.

Alternatively, a fast oscilloscope may be used to capture the analoguedrive signal and/or the drive waveform in a single cycle and provide thecaptured signals to the digital control circuit. The digital controlcircuit measures the rate of change of voltage with time, dv/dt, ofeither or both of the voltage transitions of the digitised signal. Usingequation 1, the known current of the analogue drive signal, and themeasured dv/dt, the digital control circuit determines the capacitanceof the capacitive load. The digital control circuit may also compare themaximum voltage of the digitised signal to the desired post-chargevoltage Vhigh. Similarly, the digital control circuit may compare theminimum voltage of the digitised signal to the desired post-dischargevoltage Vlow. The digital control circuit can therefore determinewhether the digitised signal overshot Vhigh and/or undershot Vlow. For acurrent drive circuit, overshooting and undershooting indicate that thecurrent pulse was too wide for the actual capacitance of the capacitiveload. The digital control circuit may also determine whether thedigitised signal oscillates at the ends of the voltage transitions, i.e.whether there is any signal ringing. Thus, the digital control circuitcan determine the current amplitude and the width of the current pulseto apply to drive that capacitive load with the desired dv/dt, andwithout causing any signal ringing or overshooting of Vhigh orundershooting of Vlow.

The digital control circuit may carry out the testing described above ata range of different temperatures. In this way, the digital controlcircuit determines the capacitance of the capacitive loads in differenttemperature ranges. Subsequently, in operation, the digital controlcircuit may receive the temperature of the printhead actuators, anddetermine the capacitance of the active printhead actuators to be thesum of the determined capacitances of those printhead actuators in thetemperature range which encompasses the measured temperature.

The digital control circuit may carry out the testing described abovewhen the printhead actuators have been active for a range of differenttime periods. In this way, the digital control circuit determines thecapacitance of the capacitive loads in different time period ranges.Subsequently, in operation, the digital control circuit may receive thetime period that the printhead actuators have been active for, anddetermine the capacitance of the active printhead actuators to be thesum of the determined capacitances of those printhead actuators in thetime period range which encompasses the measured time period.

For a voltage drive circuit, the digital control circuit compares thedigitised signal to an ideal waveform. The digital control circuitcomputes an error signal to be the difference between the ideal waveformand the digitised signal. Another test digital voltage drive signal isgenerated which is modified from the last test digital voltage drivesignal by deducting the error signal from the last test digital voltagedrive signal. This is applied to the drive circuit, sensed, digitisedand fed back to the digital control circuit. This process is performediteratively in order to produce a distorted digital voltage drive signalwhich is optimised for that capacitive load. The distorted digitalvoltage drive signal is stored for that capacitive load. The digitalcontrol circuit may perform this calibration process for everycapacitive load, and store a digital voltage drive signal to use whendriving each capacitive load. Alternatively, the digital control circuitmay store a set of different distorted digital voltage drive signals,each one of which is used when driving a range of capacitive loads. Forexample, the set may consist of four or eight distorted digital voltagedrive signals which are used to drive a total capacitive load range of0-40 nF.

The digital control circuit may carry out the testing described above ata range of different temperatures. In this way, the digital controlcircuit determines an optimised distorted digital voltage drive signalfor each of a range of capacitive loads in each of a set of differenttemperature ranges. Subsequently, in operation, the digital controlcircuit may receive the measured temperature of the printhead actuatorsand the capacitive load, and select the distorted digital voltage drivesignal to be the one optimised for the capacitive load range thatencompasses the capacitive load and the temperature range thatencompasses the measured temperature.

The digital control circuit may carry out the testing described abovewhen the printhead actuators have been active for a range of differenttime periods. In this way, the digital control circuit determines anoptimised distorted digital voltage drive signal for each of a range ofcapacitive loads in each of a set of different time period ranges.Subsequently, in operation, the digital control circuit may receive thetime period that the printhead actuators have been active for and thecapacitive load, and select the distorted digital voltage drive signalto be the one optimised for the capacitive load range that encompassesthe capacitive load and the time period range that encompasses themeasured time period.

The testing processes described above may be applied during manufactureof the printhead drive circuit. In this case, predetermined capacitancesof each capacitive load are generated, optionally at a range oftemperatures and/or time periods. Similarly, predetermined distorteddigital voltage drive signals may be generated, optionally optimised forspecific capacitance, temperature and/or time period ranges.Alternatively, or in addition, the testing processes may be applied as astartup test for a printer. Thus, the testing is applied when theprinthead drive circuit is in an operational configuration but prior toa printing operation occurring. Alternatively, or in addition, thetesting processes may be applied during a printing operation in betweenactive printing cycles.

The digital control circuit may perform testing during manufacture, atstartup and between active printing cycles of a printing operation.Suitably, the digital control circuit uses the most recent calibrationdata to determine the modifications to the digital drive signal for eachprinting cycle to match the current capacitance of the variablecapacitive load in order to generate optimised drive waveforms for thatprinting cycle. The digital control circuit may maintain a log of allthe calibration data determined for output to a system analyser. Thesystem analyser uses this data to analyse the condition of the bank ofprinthead actuators, and determine whether maintenance and/orreplacement of the printhead actuators is needed for improved operation.

The printhead drive circuit may further comprise circuitry forpreventing overshoot of Vhigh and undershoot of Vlow during active printcycles. Comparator 498, 598, 698 receives an indication of the drivewaveform voltage. The comparator may receive the amplified drivewaveform and return waveform which are sensed at the printhead actuatorend of cable 474,574,674 and 476,576,676. Alternatively, oradditionally, the comparator may receive the amplified analogue drivesignal and return waveform which are sensed at the output of the drivecircuit 470,570,670 and 472,572,672. DAC 466,566,666 receives athreshold voltage from the digital control circuit which it converts toan analogue threshold voltage which is input to comparator 398,598,698.The comparator compares the indication of the drive waveform voltage tothe threshold voltage. The output of the comparator is input to thedigital control circuit. The comparator output changes state when theindication of the drive waveform voltage crosses the threshold voltage.When the comparator changes state, the digital control circuit respondsby stopping output of the digital drive signal to the drive circuit. Thedigital control circuit may, in response to the comparator changingstate, stop generating the digital drive signal.

The digital control circuit generates the threshold voltage applied tothe comparator. The digital control circuit changes the thresholdvoltage dependent on whether the capacitive load is being charged ordischarged. The drive circuit has a non-negligible turn off time. Thus,the digital control circuit generates the threshold voltage duringcharging such that the maximum voltage of the drive waveform is Vhigh.Similarly, the digital control circuit generates the threshold voltageduring discharging such that the minimum voltage of the drive waveformis Vlow. The time taken for the drive circuit to turn off, Toff, ismeasured in advance and known to the digital control circuit. Forexample, Toff may be in the range 10-20 ns. Since the slew rate of thedrive circuits described herein is known and highly linear, the digitalcontrol circuit can accurately determine the threshold voltage Voffusing Vhigh or Vlow, Toff and the known dv/dt. FIG. 10 illustrates thatthe threshold voltage Voff is chosen for the low to high voltagetransition such that a time Toff after the digital drive signal has beendisabled, the drive waveform reaches its maximum voltage of Vhigh.Similarly, FIG. 10 illustrates that the threshold Voff is chosen for thehigh to low voltage transition such that a time Toff after the digitaldrive signal has been disabled, the drive waveform reaches its minimumvoltage of Vlow.

The use of the comparator ensures that the digital drive signal duringactive printing cycles of the printhead drive circuit does not overshootVhigh and/or undershoot Vlow even if the characteristics of theprinthead actuators have changed since the last calibration occurred,for example as a result of temperature changes. These changes cause thecapacitance of the capacitive load and the slew rate dv/dt of thechanging voltage component of the drive waveform to be different to thatexpected by the digital control circuit.

The digital control circuit may detect the elapsed time in each printingcycle at which the comparator changes state. The digital control circuitmay detect the temperature of the variable capacitive load in eachprinting cycle at which the comparator changes state. The digitalcontrol circuit may adjust the detected elapsed time for the detectedtemperature to form an adjusted detected elapsed time at which thecomparator changes state. The digital control circuit may then comparethe adjusted detected elapsed time to a threshold elapsed time. If theadjusted detected elapsed time is outside of the threshold elapsed time,the digital control circuit may determine that a fault has beendetected. This fault may be, for example, that the printhead actuatorhas short circuited or got stuck open.

The slew rate dv/dt of the rising and falling edges of the drivewaveform affects the resultant ink ejection velocity and hence the pixelplacement accuracy of the resultant printed image. Both the currentdriven and voltage driven printhead actuator driving circuits andmethods described herein provide a drive waveform having a highly linearslew rate of the desired magnitude in the drive waveform for a printingcycle which is independent of the number of printhead actuators activein that printing cycle. This enables the active printhead actuators toeject the desired volume of ink at the precise velocity desiredindependent of the number of other printhead actuators active in theprinting cycle.

If the voltage of the drive waveform undershoots Vlow and/or overshootsVhigh, this changes the total applied voltage to the printhead actuatorswhich affects the volume of ink ejected from the printhead actuators andhence changes the darkness of the printed pixel. If the printheadactuator characteristics change between calibration and an activeprinting cycle, the comparator described herein prevents theundershoot/overshoot problem during the active printing cycle byswitching the digital drive signal off to prevent theundershoot/overshoot of the drive waveform.

The printhead actuator driving circuits described herein improve themean time between failing (MTBF) of the printhead actuators by avoidingthe stress to the printhead actuators which is caused by ringing of thedrive waveform.

Undershooting of Vlow can lead to a negative drive waveform which canlatch up and damage the cold switch silicon. The printhead actuatordriving circuits described herein improve the mean time between failing(MTBF) of the cold switch silicon by avoiding undershooting of Vlow.

The current drive system described herein minimises power dissipatedwithin the drive circuitry because high power is only dissipated whenthe drive circuit is slewing its output signal between high and lowvoltages.

By optimising the drive waveform to the capacitive load of the currentprinting cycle, longer cables can be used between the drive circuit andthe bank of printhead actuators and still enable acceptable printingquality to be achieved.

Capacitor 438,538,638 reduces the dynamic range of the capacitive loaddriven. For example, it might reduce it from 400:1 (40 nF to 100 pF) to40:1 (40 nF to 1 nF). This enables easier optimisation of the drivecircuit for the full range of capacitive loads.

In the following example, the circuitry of FIG. 1 is used in electrodischarge machining (EDM). In this example, a variable capacitor isdischarged over a spark gap between an electrode and a workpiece.

FIG. 11 illustrates a known EDM arrangement. A switched voltage driver1102 generates a fast risetime pulse which propagates along cable 1104from the switched voltage driver 1102 to electrode 1106. Electrode 1106is separated from workpiece 1108 by spark gap 1110. The cable 1104 istypically long (in electrical wavelength terms, relative to the risetimeof the pulse, which could be of the order of a few to tens ofnanoseconds) and generally has a poorly defined impedance. The impedanceof the spark gap 1110 changes with electrode/workpiece geometry, whichresults in further mismatch of the source impedance to the load.

As shown in FIG. 11 , a square drive signal pulse is output fromswitched voltage driver 1102. This signal is distorted by reflectionsalong the cable 1104. The resulting drive waveform at the electrode 1106has a poorly defined dv/dt on its rising edge and ringing/overshoot.Thus, the dv/dt of the drive waveform pulse as it is applied across thespark gap can be anywhere between the maximum possible that the switchedvoltage driver can generate and zero or negative when the signal isringing.

The voltage at which the spark gap 1110 ionises is used by the controlsystem to derive the physical size of the spark gap. The ionisationvoltage depends on:

-   -   1) the dielectric strength of the insulating dielectric fluid        between the electrode and workpiece;    -   2) the shape of the electrode and workpiece (sharp points        promote lower ionisation voltages, planes and arcs, higher        voltages);    -   3) the material of the electrode and workpiece; and    -   4) the dv/dt of the applied ionisation voltage.

Because the dv/dt of known systems is highly variable, the spark gapsize determined is inaccurate.

It is known to minimise cable lengths and optimise cable types to reduceinductance and reduce impedances. However, the far end impedance changeswith capacitive loading. Also, in order for the power supply to be ableto produce short pulses for low energy microEDM or fine finishing, therise time has to be very short. This creates ringing and reflections ashigh frequency signals at significant power are transmitted along cablesthat are not of the correct impedance to match the switched voltagedrive to the load. Producing short pulses for finishing/micro workresults in signals with inconsistent and unknown dv/dt, which adverselyaffects the derivation of the spark gap size and hence, reduces theaccuracy of the resultant parts under all machining conditions.

MicroEDM power supplies have been made with R/C circuits to try tocontrol the dv/dt and achieve more accurate results. Such circuits doreduce the dv/dt, so the ringing due to reflections is reduced, but theystill suffer from varying dv/dt during the pulse risetime, and hence,are unable to derive the spark gap accurately.

The ringing and uncertainty in dv/dt at the point of ionization can bemitigated in modern systems by averaging the spark over voltage. Thisreduces the variability of the spark gap size calculation, but does notincrease its accuracy.

FIG. 12 illustrates a first exemplary current drive circuit 1200 fordriving an EDM process. Charging of variable capacitor 1222 is driven bydrive circuit 1206 under the control of digital control circuit 1202.Variable capacitor 1222 discharges providing its stored energy to sparkgap 1210 via electrode 1207. This ionizes spark gap 1210 which causes aportion of the workpiece 1208 to vaporize. Multiple charging cycles areused to produce a set of energy pulses at spark gap 1210. Thus, theworkpiece 1208 is gradually eroded to form a desired shape.

Drive circuit 1206 comprises a transformer 1212 driven by a DC supply1216 and current source 1226 via a H-bridge 1214. Transformer 1212 has asingle turn primary which is connected across H-bridge 1214. Thetransformer 1212 may be a high frequency, low leakage inductance pulsetransformer. Single polarity DC supply 1216 is connected to thetransformer 1212 through the H-bridge 1214. H-bridge 1214 allows thesingle turn primary to be alternately driven positive and negative withthe single polarity DC supply 1216. Digital control circuit 1202 outputsgate drive signals 1220 to the H-bridge 1214 which cause the H-bridge toswitch alternately positive and negative current through to thetransformer. Capacitor bank 1218 decouple the DC supply 1216 such thatvery low impedance is presented to the top of the H-bridge regardless ofthe switching frequency of the H-bridge.

The digital control circuit applies a digital drive signal to DAC 1228.DAC 1228 drives current source 1226. Current source 1226 is connected totransformer 1212 through the H-bridge 1214. Digital control circuit 1202outputs gate drive signals 1224 to H-bridge 1214 which cause theH-bridge to switch current from current source 1226 through to thetransformer. The digital control signal modifies the digital drivesignal to DAC 1228 for each charging cycle to match the voltage ramprate dv/dt of the drive waveform at the variable capacitor 1222 to thecapacitance of the variable capacitor 1222 in that charging cycle.Current from current source 1226 flows into the primary of transformer1212. The DAC 1228 generates dv/dt such that the DC current needed tocharge the variable capacitor 1222 and the magnetising current of thetransformer 1212 is sunk through the transformer primary. The currentflowing through the transformer primary causes current to flow throughthe transformer secondary, thereby charging variable capacitor 1222.FIG. 13 b illustrates the current into the primary of the transformer1212. The current is the sum of the constant charge current plus theincreasing magnetising current for the transformer. The rate of increaseof the magnetising current is dependent on the inductance of thetransformer primary and may be determined prior to operation of the EDMoperation and stored in the digital control circuit.

The spark gap 1210 is connected in parallel with the variable capacitor1222. FIG. 13 a illustrates the voltage across the variable capacitor1222, and hence the voltage across the spark gap 1210. The spark gapvoltage rises as the variable capacitor 1212 is charged until the gapionises at the point marked A on FIG. 13 a . At this point, a rapiddecay in the spark gap resistance will result in the variable capacitor1222 being discharged rapidly. The voltage across the spark gap rapidlydrops, marked B on FIG. 13 a . FIG. 13 c shows the current into thespark gap. As the variable capacitor 1222 is charged, the current intothe spark gap 1210 remains constant. When the spark gap ionises, thevariable capacitor 1222 discharges rapidly into the spark gap plasma,which causes a peak in the spark gap current, marked C on FIG. 13 c .This may cause a small peak in the current in the primary of thetransformer, marked D on FIG. 13 b , due to a lag in the current source1226 responding to the ionisation.

During ionisation, a plasma channel forms between the electrode andworkpiece. Where the plasma enters the workpiece, material is melted dueto the high power density. This forms a crater. The size of the craterdepends on the amount of energy that is dissipated in the plasmachannel.

As the current starts to flow from the variable capacitor 1222 throughthe spark gap 1210, it also flows through the transformer secondary.This current that is in excess of the charge current of the capacitor1222 depletes the flux in the transformer core rapidly. This causes thetransformer to stop delivering energy to its secondary. It may take 1-2ns for this to happen. By virtue of this, the current driven transformerdescribed is capable of delivering pulses of very low energy compared tostandard transformers. Standard transformers store considerable energyin their leakage inductance. When the spark gap ionises, the impedanceon its secondary is transformed to its primary by the square of theratio of the number of turns on the primary and secondary coils. Thiscauses the primary to have very low impedance, which pulls the dccurrent from the low source impedance voltage drive on the primary, andalso causes any stored energy on the primary (such as is stored in thedecoupling capacitors) to be delivered to the secondary. Thus, thesecondary effectively shorts out. For this reason, previous voltagedriven transformers do not deliver well controlled low energy levels.

The transformer pulse power supply of FIG. 12 generates charging pulseswhich are alternately positive and negative. This ensures that the coredoes not saturate with gradually increasing magnetic flux (by reversingthe polarity, the flux is reset on each reversal). However, some EDMoperations require either positive or negative only pulses (and some useboth). For applications where unipolar pulses are required, the EDMdrive circuit 1200 further comprises a quench diode connected inparallel with the variable capacitor 1222. This quench diode quenchesthe alternate flux reset cycles where the transformer flux is reset byreversing the polarity. In this way no pulse is generated on thetransformer secondary during the alternate flux reset cycles. FIG. 12illustrates a positive quench diode 1230 connected in parallel with thevariable capacitor 1222. This positive quench diode is switched into thecircuit by switch 1232 under the control of digital control circuit.FIG. 12 also illustrates a negative quench diode 1234 connected inparallel with the variable capacitor 1222. This negative quench diode isswitched into the circuit by switch 1236 under the control of digitalcontrol circuit.

For rough machining with very large pulse energies (for example above 5mJ), the EDM drive circuit 1200 may optionally further compriseauxiliary current sources. These auxiliary current sources drive currentinto the spark gap 1210 once it has ionized and the variable capacitor1222 has discharged. This enables more energy to be supplied to thespark gap 1210 than that stored in the variable capacitor 1222. This isillustrated in FIGS. 13 a and 13 c where it can be seen that the currentthrough the spark gap and the voltage across the spark gap remain atelevated values at points E and F after ionisation compared to theirpre-charge values. FIG. 12 illustrates a current source 1238 which isconnected to supply 1242 and driven by DAC 1240 under the control of thedigital control circuit 1202. The current source 1238 is coupled to thespark gap 1210 by FET switch 1244 and diode 1246. FET switch 1246operates under the control of the digital control circuit 1202. Thedigital control circuit 1202 causes a positive pulse to be applied tothe spark gap 1210 by driving the FET switch 1246 ON for positive cyclesand driving the DAC 1240 with a DC value or a profiled waveform for thecurrent profile to be driven into the spark gap.

FIG. 12 also illustrates a current sink 1248 which is connected tosupply 1250 and driven by DAC 1252 under the control of the digitalcontrol circuit 1202. The current sink 1248 is coupled to the spark gap1210 by FET switch 1254 and diode 1256. FET switch 1254 operates underthe control of the digital control circuit 1202. The digital controlcircuit 1202 causes a negative pulse to be applied to the spark gap 1210by driving the FET switch 1254 ON for negative cycles and driving theDAC 1252 with a DC value or a profiled waveform for the current profileto be driven into the spark gap.

The capacitance of the spark gap 1210 varies with the workpiecegeometry. FIGS. 14 a, 14 b and 14 c illustrate this for wire EDM(w-EDM). These figures illustrate a side view in which wire 1402 isguided by wire guide dies 1404 and 1406 and erodes workpiece 1408. Thecapacitance of the spark gap increases as the length of the wire that isin close proximity to the workpiece increases. In both FIGS. 14 a and 14b , the wire approaches the workpiece from free space. In FIG. 14 b ,the length of wire in close proximity to the workpiece is x2. In FIG. 14a , the length of wire in close proximity to the workpiece is x1. x2>x1,therefore the capacitance of the spark gap in FIG. 14 b is greater thanthat in FIG. 14 a . In FIG. 14 c , the wire is surrounded by material asit plunges through the solid material of the workpiece. As the wiremoves from the left hand side to the right hand side of FIG. 14 c , theoverlap between the wire and the workpiece decreases, and hence thecapacitance of the spark gap decreases during the machining process.

FIGS. 15 a-g illustrate how the spark gap 1210 varies with workpiecegeometry for plunge erosion. FIGS. 15 e and 15 f illustrate that as theseparation between the electrode 1502 and the workpiece 1508 is reduced,the spark gap capacitance increases. In FIG. 15 b , the area of theelectrode 1502 in close proximity to the workpiece 1508 is greater thanfor FIG. 15 a , and hence the spark gap capacitance of FIG. 15 b isgreater than that for FIG. 15 a . In FIG. 15 c , the electrode of FIG.15 a has plunged into workpiece 1508. Similarly, in FIG. 15 d , theelectrode of FIG. 15 b has plunged into workpiece 1508. The capacitanceof the spark gap when the electrode plunges into the workpiece isgreater than when it is approaching the workpiece. FIG. 15 g shows anelectrode 1502 approaching the side of a workpiece 1508 that is tapered.It can be seen that as the electrode moves from left to right in FIG. 15g , the spark gap capacitance will reduce due to the smaller area ofcontact. As in the w-EDM example above, this effect is more pronouncedwhen the electrode is ploughed through solid material and most of theperiphery of the electrode is in close proximity to the workpiece.

The workpiece geometry is known. Changes to the surface area between theelectrode and the workpiece as the machining process occurs may bemodelled in advance. Thus, the digital control circuit 1202 may beprovided with predetermined capacitances of the spark gap for eachcharging cycle in accordance with the known and changing workpiecegeometry. The digital control circuit then modifies the digital drivesignal applied to the DAC 1228 in order match the slew rate of thevoltage of the rising edge of the EDM pulse to the capacitance of thespark gap in that charging cycle. In this way, the digital controlcircuit 1202 maintains a constant slew rate dv/dt of the drive waveformwhilst charging the variable capacitor 1222.

During operation, as the capacitance of the spark gap increases, thesize of the variable capacitor 1222 is reduced under the control ofdigital control circuit 1202, such that the total capacitance isregulated. This ensures that the dv/dt of the drive waveform remainsconstant and thus that the energy in each EDM pulse is constant, andhence that the surface finish of the workpiece remains constant.

The digital control circuit 1202 may modify the digital drive signal toDAC 1228 for each charging cycle of the variable capacitor 1222 inaccordance with a predetermined capacitance of the spark gap for thatcharging cycle as described above.

The EDM drive circuit 1200 may further comprise a feedback circuit.Voltage feedback may be accomplished with a potential divider 1260connected in parallel with the variable capacitor 1222. The output ofthe potential divider 1260 is input to a differential amplifier 1262.The output of differential amplifier 1262 is input to ADC 1264. ADC 1264samples the voltage signal and outputs the sampled signal to digitalcontrol circuit 1202. The output of differential amplifier 1262 is inputto peak detector circuit 1266 and spark detector circuit 1270. Sparkdetector circuit 1270 detects a spark when it detects a negative dv/dt.This signifies that the spark gap has been ionized. The output of peakdetector circuit 1266 is input to ADC 1268. ADC 1268 samples the voltagesignal when a spark is detected by the spark detector circuit. ADC 1268outputs the sampled signal to digital control circuit 1202. Sparkdetector circuit 1270 outputs to digital control circuit 1202.

Current feedback may be accomplished with a sense resistor 1272connected in series between the variable capacitor 1222 and the sparkgap 1210. Alternatively, current feedback may be accomplished with an RFcurrent transformer. The signal at either side of the sense resistor1272 is fed to differential amplifier 1274. Differential amplifier 1274amplifies the current signal and outputs the amplified signal to ADC1276. ADC 1276 samples the current signal and outputs the sampled signalto digital control circuit 1202.

An alternative to the spark detector circuit 1270 is to sample theoutput of the differential amplifier 1274 with a comparator set to athreshold just above the charging current of the spark gap. Thiscomparator will then trigger when the spark gap ionizes and the currentinto the gap increases rapidly.

The digital control circuit determines the rate of change of voltagewith time of the sensed signals during charging of the variablecapacitor. Using equation 1, the digital control circuit determines thecapacitance of the variable capacitive load in the charging cyclemeasured. The digital control circuit modifies the digital drive signalto the DAC 1228 in dependence on the determined capacitance and themeasured dv/dt. The digital control circuit fine tunes the incident edgedv/dt of the drive waveform. This enables the digital control circuit tocompensate for changes in capacitance of the spark gap. It also enablesthe digital control circuit to compensate for other undefined changessuch as changes in the dielectric constant of the insulting dielectricand unexpected differences in workpiece geometry. The digital controlcircuit may determine the capacitance of the spark gap on each chargingcycle, and use this capacitance to modify the digital drive signal ofthe next charging cycle in order to modify the current of the analoguedrive signal output from the drive circuit.

On detecting a spark at the spark detection circuit 1270, the digitalcontrol circuit 1202 may stop outputting the digital drive signal to thedrive circuit 1206. Alternatively, or in addition, the digital controlcircuit 1202 may control an auxiliary current source, such as currentsource 1238 or current source 1248 to turn ON in order to cause a longerand/or higher power drive waveform pulse to be applied to the spark gap.

The digital control circuit may output the sensed signals to a systemanalyser. The system analyser may analyse the sensed signals so as tocategorise the pulse type of each charging cycle. Exemplary pulse typesare: open circuit, shorted, runt (low amplitude pulse) and normal rangeEDM pulse. The system analyser may detect a fault in the EDM drivecircuit dependent on the pulse types detected over an EDM operation. Forexample, if there are more than a threshold number or percentage ofshorted pulse types or runts, the system analyser may determine a fault.This fault may be that the spark gap is too small. This fault may bethat the flushing is not adequate. If there are more than a thresholdnumber or percentage of open pulse types, the system analyser maydetermine a fault. This fault may be that the spark gap is too large.

The digital control circuit 1202 uses the peak voltage detected by thepeak detector circuit 1266 to be the ionisation voltage. The digitalcontrol circuit 1202 determines the spark gap size from the ionisationvoltage. The spark gap size is proportional to the ionisation voltage.FIG. 13 shows that use of an EDM drive circuit of the type describedwith reference to FIG. 12 results in generation of a drive waveform witha linear slew rate on the rising edge of the ionisation pulse. Since thedv/dt is linear, the spark over voltage measured is a more accuraterepresentation of the spark gap size than the prior art systemsdescribed. Since the gap size calculation is more accurate than priorknown systems, the digital control circuit maintains the gap size moreaccurately than those prior systems. This leads to the machinedworkpiece being more geometrically accurate.

The digital control circuit 1202 may dynamically modify the digitaldrive signal input to the drive circuit 1206 to cause the drive waveformto deliver the desired pulse energy and discharge voltage to thevariable capacitor 1222 and spark gap 1210 according to the capacitanceof the spark gap. The capacitance of the spark gap may vary as a resultof any one or more of the following parameters: the known electrode andworkpiece geometries, the desired spark gap, the desired surface finish,and parasitic capacitances of the system. The digital control circuit1202 modifies the drive waveform in each charging cycle to ensure arepeatable, known dv/dt is applied across the spark gap in each chargingcycle despite changing capacitive load conditions. By applying a linearknown dv/dt, the spark gap calculation is more accurate, the surfacefinish quality of the machined product is better, and the machinedproduct is more geometrically accurate compared to prior known systems.

The EDM circuit 1200 may further comprise circuitry for preventing thevoltage across the variable capacitor from overshooting a maximumdesired voltage, Vhigh, during a charging cycle. This circuitry ensuresthat the maximum gap voltage is not exceeded. Should the spark gap notionise when the maximum desired voltage is applied, the circuitry causesthe erosion cycle to be aborted. This circuitry may comprise acomparator (not shown on FIG. 12 ) which receives the sensed voltagesignal from differential amplifier 1262. The digital control circuit1202 generates a threshold voltage which it outputs to a DAC. The DACconverts the threshold voltage to an analogue threshold voltage which itinputs to the comparator. The comparator compares the sensed voltagesignal to the analogue threshold voltage. The output of the comparatorchanges state when the sensed voltage signal crosses the analoguethreshold voltage. The comparator output is input to the digital controlcircuit 1202. When the comparator changes state, the digital controlcircuit stops outputting the digital drive signal to the drive circuit1206. The digital control circuit may, in response to the comparatorchanging state, stop generating the digital drive signal.

The drive circuit has a non-negligible turn off time, Toff. The digitalcontrol circuit generates the threshold voltage in dependence on theknown linear slew rate dv/dt, and the turn off time Toff, such that themaximum voltage of the drive waveform is Vhigh. Vhigh may be chosen tobe less than the maximum open circuit voltage. This ensures that theionisation voltage does not exceed the maximum open circuit voltage.When an open circuit occurs, the comparator senses the voltage hasexceeded the threshold voltage, and stops the digital control circuitfrom driving the drive circuit. Thus, the cycle is aborted. Thisprevents subsequent ionisation of the spark gap from being used in thecalculation of the spark gap size.

FIGS. 16 a and 16 b illustrate how the EDM drive circuit of FIG. 12 canoperate with the same energy per EDM pulse over very large ranges ofspark gap size, and hence, gap capacitance and ionization voltage.Conventional systems, when operating with large spark gaps and largeionization voltages, are unable to operate under these conditions whilstalso delivering very low energy per EDM pulse. This is due to theparasitic capacitances in the voltage switching elements.

The EDM drive circuit of FIG. 12 compensates for the variable spark gapcapacitance as described above. The high frequency, low leakageinductance pulse transformer stores very little energy and allows a highvoltage. These features of the EDM drive circuit enable low energy EDMpulses to be produced.

In FIG. 16 a , the electrode 1601 on the left is separated from itsworkpiece 1602 by a spark gap size of 10 μm. The electrode 1601 on theright is separated from its workpiece 1602 by a spark gap size of 30 μm.Thus, the capacitance of the spark gap is much greater for the setup onthe left than the right. The ionisation voltage of the spark gap is muchgreater for the setup on the right than the setup on the left. However,since the digital control circuit modifies the drive signal to match thetime dependent voltage component of the drive waveform to thecapacitance of the spark gap in that charging cycle, the slew rate dv/dtof the drive waveform is the same for the machining of both theworkpiece on the left and the workpiece on the right. The spark gapionisation voltage is proportional to the spark gap size. Thus, the samesurface finish is achieved on both workpieces. The spark gap ionisationvoltage on the left may be 100V, and on the right 300V. The capacitiveload on the left may be 1 nf, and on the right 110 pF. The digitalcontrol circuit modifies the current of the drive waveform to be 1.0 Ampon the left, and 0.11 Amp on the right. This causes both sides to have aconstant dv/dt of 1000V/μs and a constant discharge energy of 5 μJ. Asthe spark energy can be maintained at a low level over large spark gapsizes, surfaces can be finish machined with better flushing efficiency.

FIG. 16 b illustrates the use of the EDM drive circuit of FIG. 12 inrough machining. A very high pulse energy is used which generates arougher surface on the workpiece. In this example, the spark gapionisation voltage may be 300V and the capacitive load 22 nF. Thedigital control circuit modifies the current of the drive waveform to be22.0 Amp. This causes a constant dv/dt of 1000V/μs, which is the same asthat achieved for the fine machining of FIG. 16 a . A larger dischargeenergy of 1 mJ creates the rougher surface of the workpiece. Thus, thesame EDM drive circuit is able to operate in the nJ and mJ pulse energydomains.

FIGS. 17 a and 17 b illustrate an exemplary transformer which can beused as transformer 1212 of the EDM drive circuit. Transformer 1212 canalso be used as part of a 3D printing or power supply (e.g. a switchedmode power supply) drive circuit. This transformer is a high frequency,low leakage inductance pulse transformer.

The transformer primary has primary windings 1712 which are connected toarms 1702 that pass through a magnetic material. The magnetic materialmay comprise one or more toroidal cores 1704. This is easily seen inFIG. 17 b , which is a cross sectional view through one of the tube armsof FIG. 17 a . Passing the primary through the centre of the toroidalmagnetic cores ensures the best coupling of the primary to the cores andresults in the smallest stray field, which contributes to leakageinductance. The arms 1702 may be copper tubes. The copper tubes have alarge surface area. The skin effect constrains the current flow to theoutside of conductors at high frequency, increasing their impedance. Byutilising large surface areas on the inside and the outside of thecopper tubes, the skin effect is reduced. This allows the electricalpath to have a low inductance at high frequency. The arms 1702 may beconnected by a shorting bar 1706.

The transformer secondary has secondary windings 1714 connected tosecondary conductors 1708 which pass through the inside of the primarycopper tube. This enables greater coupling of primary to secondary asthey both experience the same magnetic flux. The secondary conductorsmay be planar. The secondary conductors may comprise three planarlayers, in which a copper foil layer is attached to either side of aPCB. The planar construction of the secondary conductors reduces theinductance of the secondary conductors at high frequency. The secondaryconductors may be perpendicular to the side walls of the primary. Thiscauses the smaller height dimension (which may be in the range 35-100μm) of the secondary inductor to be facing the side wall of the primaryrather than the larger width (which may be in the range 2-5 mm)dimension. This causes a lower capacitive coupling between the primaryand the secondary. The secondary conductors 1708 may be arranged in aradial formation separated by insulator 1710 inside the primary. Thesecondary conductors may be equally spaced apart from each other withinthe primary tube. The larger face of each secondary conductor is offsetfrom parallel with the larger face of the secondary conductors adjacentto it. For example, in a 6 turn secondary as depicted in FIG. 17 , thelarger faces of the secondary conductors are offset at an angle of 60°to each other. This reduces the intra winding parasitic capacitance to avery low level, further reducing the stored energy.

Use of the transformer of FIGS. 17 a and 17 b in the EDM drive circuitof FIG. 12 enables the pulse energy of each EDM pulse to be in the range5 nJ to 50 mJ. This is a larger dynamic range than known systems. Thetransformer when used in the EDM drive circuit is able to generate pulserepetition rates of greater than 1 MHz.

FIG. 18 illustrates a further EDM drive circuit. This is the same as theEDM drive circuit of FIG. 12 except that the transformer 1812 of thedrive circuit 1806 is a push-pull wound transformer of the type known inthe art. This EDM drive circuit operates as described with respect toFIG. 12 , but transformer 1812 has a higher leakage inductance thantransformer 1212, and hence the EDM drive circuit of FIG. 18 cannotproduce the very low EDM pulses of the drive circuit of FIG. 12 . TheEDM drive circuit of FIG. 18 may further comprise auxiliary currentsources (not shown) to drive large pulse energies, as described withrespect to FIG. 12 .

FIG. 19 illustrates a further EDM drive circuit. This is the same as theEDM drive circuit of FIG. 12 except that the drive circuit 1906 directlycouples two current sources 1982 and 1986 to the variable capacitor 1922under the control of the digital control circuit 1902. The slewingcircuit of the drive circuit 1906 is connected and operates as describedwith respect to the slewing circuit of FIG. 4 . The EDM drive circuit ofFIG. 19 may further comprise auxiliary current sources (not shown) todrive large pulse energies, as described with respect to FIG. 12 .

FIG. 20 illustrates a further EDM drive circuit. This is the same as theEDM drive circuit of FIG. 12 except that the drive circuit 2006 directlycouples a single current source 2094 to the variable capacitor 2022under the control of the digital control circuit 2002. The slewingcircuit of the drive circuit 2006 is connected and operates as describedwith respect to the slewing circuit of FIG. 5 . The EDM drive circuit ofFIG. 20 may further comprise auxiliary current sources (not shown) todrive large pulse energies, as described with respect to FIG. 12 .

The response time of the current sources of FIGS. 19 and 20 precludeoperation down to the 5 nJ level. This is because the energy in thecurrent source pass transistors that is released into the spark gap whenit ionizes is not interrupted as quickly as when the high frequency, lowleakage inductance pulse transformer 1202 of FIG. 12 is used. Thus, ahigher energy has flowed into the spark gap before it is halted.

The EDM drive circuits described herein produce an EDM pulse with a morecontrolled and lower dv/dt, with no overshooting of the maximum desiredionisation voltage. This reduces signal ringing and also reduces thesignal reflections suffered by known systems. This enables longer cablesto be used.

The EDM drive circuits described herein enable very low energy EDMpulses to be generated. These very low energy EDM pulses have ionisationvoltages of up to 300V. Thus, fine finishing is possible without havingto use very small spark gaps as in known systems. By using a largerspark gap, flushing of the eroded region is more effectively carriedout, which results in higher quality surface finishes.

In the following example, the circuitry of FIG. 1 is used in 3D metalprinting.

Known 3D metal printing is implemented using a laser sintering process.The laser sintering process uses a bed of powder which is scraped overthe top surface of the workpiece and then melted or fused to the surfacebelow, using a focused laser.

Advances in laser technology have made it possible to use solidstate/dye lasers, rather than gas lasers and q-switched crystal laserssuch as Neodymium doped glass. The former are lower cost, lowermaintenance and more robust than the latter. This has reduced the costof the machines and allowed the technology to gain more widespread usein the industry, but laser sintering machines are still high costinstruments and the parts made on them are very much more expensive thanparts made by conventional subtractive manufacturing. Some attempts havebeen made to improve the precision of the laser sintering process byadding high speed milling spindles to sintering machines, so thatcritical areas can be machined in process.

Research into the use of 2D printheads to print plastics and even metalsin suspension is underway, but the later requires the part to be firedin an oven or otherwise heated in order to remove the suspension medium.A third process may be required if low porosity is required to fill thevoids created by this approach.

FIG. 21 illustrates a 3D metal printer architecture which utilises thedrive circuitry of FIG. 1 . EDM drive circuitry 2102 uses the drivecircuitry of FIG. 1 to generate energy pulses which generate materialsource voxels 2104. A fusion pulse power supply 2106 also uses the drivecircuitry of FIG. 1 to control charging of a variable fusion energystorage capacitor which is discharged into the plasma flow to fuse thevoxel onto the workpiece.

Inert gas, such as nitrogen or argon, is fed to the top of the printhead2108. This gas is fed in at a controlled pressure and mass flow rate.The gas flows along a quartz or other insulating tube 2110 to the EDMsource cell 2112. Two wires of source material 2114 are fed coaxiallyinto the source cell, leaving a spark gap (of the order 10-30 um) in thecentre. The EDM power supply 2102 generates an EDM pulse for every voxelof material that is to be printed. The desired size of the voxel isdependent on the precision of print resolution needed in that printingcycle. The size of the voxel is determined by the energy dischargedbetween the two wires 2114. The energy discharged between the two wiresis controlled by the EDM drive circuit, as described in detail above.The spark gap size is determined precisely as described above. Thedigital control circuit of the EDM drive circuit modifies the digitaldrive signal to the drive circuit as described above to maintain aconstant voltage ramp rate dv/dt whilst the variable capacitor ischarging to ensure that the spark gap between the wires remains aconstant size. The spark gap size is maintained by feeding the sourcematerial wires 2114 into the EDM source cell 2112 as the source materialis eroded to form the voxels. The spark gap can be maintained in thecentre of the EDM source cell using a charge coupled device (CCD) sensor(not shown) which detects photons which are emitted as the gas isionized. The digital image output by the CCD sensor is processed todetermine where the start and end point of each discharge is in the EDMsource cell.

The voxel is then forced down the tube by the flow of the gas and bygravity. The voxel passes through an RF capacitively coupled plasmageneration cell 2116. Here, the gas is ionized between electrodes 2120by energy supplied from the HF AC power supply 2118. The plasma remainsin a non-thermal equilibrium state.

The voxel proceeds further down through the printhead where the plasmais constrained and focused by magnetic lenses 2122. These lenses confinethe plasma to the centre of the printhead. The source material voxelstend to align to the centre of the tube as the pressure is lowest there.This is because the velocity of the gas/plasma is highest in the centreof the tube. The velocity of the gas/plasma is lower at the tube wallsdue to friction.

The plasma and material voxels then pass through output electrode 2124.Electrode 2124 may be made of graphite. Suitably, electrode 2124 iswater cooled and gas cooled through high pressure application of theinert print gas (for example nitrogen or argon) through the porousstructure of the electrode.

The plasma column converges as a result of the magnetic lenses. Theworkpiece 2126 is positioned such that the focal point 2128 of theplasma is on the surface of the workpiece, where the source material isto be fused.

Energy to fuse the source material to the workpiece is supplied by thefusion pulse power supply 2106. The desired energy supplied in eachenergy pulse is dependent on the source material volume of the voxel.The energy is supplied using the drive circuitry of FIG. 1 by a variablecapacitor which is charged to a high voltage (such as 200V-2 KV) andthen discharged into the plasma over a short period (for example 2-20ns). The current during discharge may reach up to 50-500 Amps. Thedigital control circuit of the drive circuitry modifies the digitaldrive signal to precisely regulate the voltage discharged at the fusionpulse power supply 2106 to be the desired voltage regardless of pulserepetition rate. The peak power of the fusion pulse power supply 2106may be in the range 10-200 KW.

FIG. 22 illustrates an exemplary fusion pulse power supply drivecircuit. Digital drive circuit 2202 generates a digital drive signalwhich drives drive circuit 2206 to produce an analogue drive signal forcharging the variable capacitor 2222 which is subsequently dischargedbetween the electrode 2207 and workpiece 2208.

Drive circuit 2206 comprises a keeper circuit comprising DAC 2280,voltage amplifier 2282 and resistor 2290. The digital drive signal fromdigital control circuit 2202 is input to DAC 2280. The output of DAC2280 is input to voltage amplifier 2282. The output of the voltageamplifier 2282 is input to resistor 2290. The output of resistor 2290 isconnected to the output of the drive circuit 2206. The keeper circuitdrives a static voltage component of the analogue drive signal outputfrom the drive circuit 2206. The HV voltage amplifier 2282 is driven tothe terminal voltage of variable capacitor 2222. This ensures that thevariable capacitor 2222 remains at the desired discharge voltageregardless of any leakage currents in the fusion pulse power supplydrive circuit.

Drive circuit 2206 comprises a slewing circuit comprising DAC 2228,current source 2286, DC voltage supply 2288 and switch 2284. DC supply2288 may be variable. DC supply 2288 may have a range from 100V to 2 KV.DC supply 2288 may have a range from 100V to 1 KV. DAC 2228 receives adigital drive signal from digital control circuit 2202. Current source2286 is connected to DC supply 2288, and is driven by DAC 2228 under thecontrol of digital control circuit 2202. Switch 2284 is between theoutput of the current source 2286 and the output of the drive circuit2206.

The analogue drive signal output from the drive circuit 2206 chargesvariable capacitor 2222. There is a switch 2286 between the variablecapacitor 2222 and the electrode and workpiece. To initiate a capacitorcharge cycle, switch 2284 is closed, switch 2286 is opened and currentsource 2286 is driven with current from DAC 2228 under the control ofdigital control circuit 2202. The capacitor 2222 charges to its terminalvoltage. This is shown in FIG. 23 a , by the linear voltage rampslabelled A.

Once the terminal voltage has been reached, the voltage amplifier 2282maintains this voltage until the source material reaches the workpiece.This is marked B on FIG. 23 a . The time to maintain the terminalvoltage is determined by the digital control circuit, or received by thedigital control circuit from a system controller. The time for the voxelto reach the workpiece 2126 is determined from the known velocity of thegas in the printhead and the distance between the spark gap 2112 in thesource material and the workpiece 2126. The terminal voltage ismaintained for this time period after a timestamp of the EDM pulsespark.

Under the control of digital control circuit 2202, the switch 2284 isthen opened coincident with the switch 2286 being closed. This connectsthe charged capacitor 2222 between the output electrode and theworkpiece. The charge in the capacitor 2222 flows through the lowresistance of the plasma, raising its temperature rapidly. FIG. 23 ashows the rapid discharge of the capacitor, marked C. With very lowinductance, planar interconnect between the fusion pulse power supplyand the output electrode and workpiece, the current rises rapidly. Forexample, 1 KV discharged across 10 Ohms may result in 100 Amps. Thecurrent rises to this level in a few nanoseconds. FIG. 23 b shows therapid rise in voltage across the plasma, and FIG. 23 a shows the rapidrise in current through the plasma. The charge in the capacitor flowsthrough the plasma, where the stored energy is dissipated.

The current through the plasma in each charging cycle is sensed and fedback to the digital control circuit 2202. Series resistor 2272 isconnected between switch 2286 and the electrode 2207. The signal ateither side of the series resistor 2272 is fed to differential amplifier2274. Alternatively, an RF current transformer may be used to detect theplasma current. The output of the differential amplifier 2274 is inputto peak detector 2266. The output of the peak detector 2266 is input toADC 2268. The output of ADC 2268 is input to digital control circuit2202. Peak detector 2266 detects the peak current which is digitized byADC 2268 and output to the digital control circuit 2202. The digitalcontrol circuit calculates the plasma resistance of that charging cyclefrom the peak current and the known voltage that the variable capacitor2222 has been charged to in that charging cycle. The energy delivered tothe CCP plasma generation cell by electrodes 2120 is modulated by the ACpower supply 2118 in dependence on the calculated plasma resistance, soas to maintain a constant plasma resistance through the plurality ofcycles of the printing operation.

The volume of each source material voxel to be printed is provided tothe digital control circuit. The rate at which the voxels are to beprinted is also provided to the digital control circuit. The voxelvolume may differ from charge cycle to charge cycle. The digital controlcircuit modifies the capacitance of the variable capacitor 2222 in eachcharge cycle to change the terminal voltage of the capacitor such thatit stores the desired energy for the volume of source material in thevoxel of that charge cycle. The digital control circuit also modifiesthe digital drive signal sent to the drive circuit 2206 in each chargingcycle to cause the keeper circuit to generate a keeper voltage of theterminal voltage of the capacitor for that charging cycle. The digitalcontrol circuit also modifies the digital drive signal sent to the drivecircuit 2206 in each charging cycle to cause the slewing circuit togenerate an analogue drive signal having a time dependent voltagecomponent which has a linear slew rate of the desired magnitude to causethe capacitor 2222 to be charged in time to generate a pulse at thedesired time. If the print rate is desired to be increased, then thedigital control circuit speeds up the charging cycles by increasing theramp rate dv/dt to charge the capacitor 2222 more quickly, and switchingswitches 2284 and 2286 more quickly to discharge the capacitor 2222 intothe plasma more quickly. The digital control circuit thereby causes thedesired amount of energy to be delivered to the focal point of theplasma flow at the desired time for each fusion pulse to melt and fusethe volume of the source material in the voxel of the current chargecycle to the workpiece.

The fusion pulse power supply drive circuit 2200 may further comprisecircuitry for preventing the voltage across the variable capacitor fromovershooting the target high voltage, Vhigh, during a charging cycle.This circuitry may comprise a comparator (not shown on FIG. 22 ) whichreceives a sensed voltage signal. The voltage signal may be sensed asdescribed with respect to the EDM pulse power supply drive circuit ofFIG. 12 . The digital control circuit 2202 generates a threshold voltagewhich it outputs to a DAC. The DAC converts the threshold voltage to ananalogue threshold voltage which it inputs to the comparator. Thecomparator compares the sensed voltage signal to the analogue thresholdvoltage. The output of the comparator changes state when the sensedvoltage signal crosses the analogue threshold voltage. The comparatoroutput is input to the digital control circuit 2202. When the comparatorchanges state, the digital control circuit stops outputting the digitaldrive signal to the drive circuit 2206. The digital control circuit may,in response to the comparator changing state, stop generating thedigital drive signal.

The drive circuit has a non-negligible turn off time, Toff. The digitalcontrol circuit generates the threshold voltage in dependence on theknown linear slew rate dv/dt, and the turn off time Toff, such that themaximum voltage of the drive waveform which charges the capacitor 2222is Vhigh. Vhigh may be chosen to be the terminal voltage for thecapacitor for that charging cycle. Thus, by preventing Vhigh from beingovershot, the amount of energy delivered to fuse the source material tothe workpiece does not exceed the desired amount.

The EDM power supply 2102 may operate using one of the EDM drivecircuits described above. The digital control circuit modifies thecapacitance of the variable capacitor in each printing cycle such thatits terminal voltage matches the energy to be discharged in the sparkgap to cause a voxel of source material of the desired size to begenerated for that printing cycle. The digital control circuit modifiesthe digital drive signal for each printing cycle so as to match the timedependent voltage component of the drive waveform to the capacitance ofthe variable capacitor for that printing cycle. Overshoot circuitry canbe used to prevent overshooting of the terminal voltage of thecapacitor. The digital control circuit modifies the digital drive signalfor each printing cycle to ensure the dv/dt of the ionising pulseapplied between the electrodes is stable and repeatable. This enablesthe spark gap size to be determined more accurately than known EDMsystems. This ensures that the source cell discharge region isstationary and the material voxels all start in the printhead from thesame location, which improves the accuracy of the printed parts comparedto known EDM systems.

The EDM power supply drive circuit and the fusion pulse power supplydrive circuit described herein ensure that, for each printing cycle, avoxel of the desired volume of source material is generated, and thatthe correct amount of energy to fuse that voxel to the workpiece isapplied to the plasma at the correct time. The resulting printed partthus has the precise geometric form desired, and is also repeatable. Theresulting printed part also has better metallurgical qualities comparedto laser sintering. This is because the plasma flow is focussed to alarger point on the surface of the workpiece than with laser sintering.This avoids the rapid and localised heating and cooling effect of lasersintering.

The voxel size can be varied on each charging cycle using the EDM powersupply drive circuit described herein. The same EDM power supply drivecircuit may be able to generate a voxel between 1 μm³ and 10,000 μm³.This enables, in the same printing operation, larger voxel sizes to beused to quickly print large areas of low geometric complexity, andsmaller voxel sizes to be used to print areas of high geometriccomplexity, small detail geometry or if higher accuracy is desired.

The use of a plasma in non-thermal equilibrium allows precise control ofthe fusion energy such that the source material can be fused to theworkpiece regardless of its melting point, from close to roomtemperature up to thousands of Kelvin.

The 3D metal printer described herein uses source material in wire form,whereas known sintering processes use metal powder. Metals are morereadily available in wire form. One wire spool is easily and quicklyexchanged for another. Thus, the 3D printer described herein enablesprint materials to be changed more easily than in sintering processes.

In a further example, the 3D metal printer architecture of FIG. 21 ismodified using a laser to refine the fusion point of the sourcematerial. The EDM cell described generates the source voxels. The plasmatransports the source material to the workpiece. This approach allowsthe plasma to provide a larger area of heating to improve themetallurgical qualities of the resultant part, while the laser providesthe high power and precision focusing that results in higher precisionof the final printed part.

In a further example, the fusion pulse power supply drive circuit 2200may comprise a plurality of drive circuits 2206, each one driving acorresponding variable capacitor 2222 whose output can be switched intoa circuit to discharge into the plasma. These drive circuits arearranged in parallel and controlled by the digital control circuit 2202to be switched in quick succession. The capacitor driven by the firstdrive circuit discharges into the plasma reducing the plasma resistanceto a first resistance (for example 10 Ohms). The capacitor driven by thesecond drive circuit then discharges into the plasma reducing the plasmaresistance to a second resistance (for example 1 Ohm). The first andsecond discharges may have similar energies, but the second dischargereduces the resistance of the plasma more quickly due to the highercurrent. This approach allows the power dissipated to be arbitrarilyincreased, and is therefore used when the application/material requiresvery high power levels in order to be fused to the workpiece.

It is described above how using a current source 1226 having highimpedance to charge a variable capacitive 1222 load via a transformer(specifically the transformer of FIGS. 17 a and 17 b ) allows thecharging to be effected at a precise rate (dv/dt). It also has theadvantage that when the voltage across the variable capacitance 1222(and therefore, the spark gap 1210) reaches the ionisation potential forthe prevailing conditions between the spark gap electrodes 1207, 1208,the dielectric breaks down and there is a very rapid and exponentialincrease in conductivity. In such a situation, a voltage driven powersupply (as opposed to current driven using current source 1226), havinglow impedance, would pass significant amounts of energy from the lowimpedance supply on the primary side of transformer 1222, through thetransformer 1212, to the spark gap 1210 on the secondary side oftransformer 1222. In order to stop this from happening, a control systemwould have to sense the increase in current and react to it, switchingoff or isolating the low impedance supply to the primary, to limit theenergy transferred to the spark gap 1210. This is difficult toaccomplish with comparators and switching transistors in a short timeand, in this time, unwanted energy may be dumped into the ionised sparkgap, affecting the machined surface finish (for EDM applications) andthe size of the source material voxel (for 3D printing applications).Using a current source 1226, as soon as spark gap 1210 ionises and theimpedance seen across transformer 1212 secondary drops (typically inless than 1 ns), current source 1226 will respond by increasing itsimpedance to maintain the commanded current flow. This results in thevoltage across the primary rapidly dropping, in turn stopping the flowof energy from primary side to the secondary side. The magnetic fluxwithin the transformer 1222 core drops rapidly as it is starved ofmagnetising current and it no longer functions as a transformer as allenergy transfer is extinguished.

FIG. 24 a illustrates a further example of a drive circuit, similar tothe EDM drive circuit 1200 of FIG. 12 . In contrast to drive circuit1200 comprising a current source 1226, drive circuit 2400 comprises avoltage source 2426. Voltage source 2426 has a low impedance. Voltagesource 2426 is configured to apply a voltage to the primary side oftransformer 2412. Voltage source 2426 may be configured to apply aconstant voltage, irrespective of the current delivered by the voltagesource 2426. In other words, the voltage source 2426 may be configuredto supply a range of currents by applying a constant voltage.Transformer 2412 may be of the kind described above with reference totransformer 1212. A capacitor 2422 may be connected to the secondaryside of transformer 2412. Capacitor 2422 has a fixed capacitance, incomparison to the variable capacitor 1222. The fixed capacitance may beprovided by a fixed capacitance by having a fixed geometry. A current isoutput by the drive circuit 2400 at the secondary side to drive a load.The load may comprise an electrical load, for example a circuit thatdraws power. The output current may be a direct current.

Switching elements 2406, 2408 may be provided to enable voltage source2426 to alternately provide positive and negative current through thetransformer 2412. Switching elements 2406, 2408 may be respectivelyconnected to the ends of transformer primary and ground (or a rail heldat a reference voltage). Voltage source 2426 may be connected to thetransformer primary part way along in a centre tap/tapped (CT)arrangement. A CT arrangement allows two fewer switching elements to beused, as can be seen from the comparison of the primary side shown inFIG. 24 a (comprising two switching elements) and the primary side shownin FIG. 12 (comprising four switching elements in an H-bridgearrangement). If switching element 2406 is closed and switching element2408 is open, a current will pass through the transformer 2412 in afirst direction. If switching element 2408 is closed and switchingelement 2406 is open, a current will pass through the transformer 2412in a second direction, the second direction being opposite to the firstdirection. Switching elements 2406, 2408 allow the primary to bealternately driven positive and negative by a single polarity voltagesupply 2426. Switching elements 2406, 2408 may comprise transistors, forexample field-effect transistors.

The capacitor 2422 shown in FIG. 24 a may be connected to the secondaryside of transformer 2412 in a CT arrangement. The centre tap isconnected to a first side of capacitor 2422 and a second side ofcapacitor 2422 is connected to both ends of the transformer secondaryvia diodes. This CT arrangement acts to rectify the direct currentoutput by the drive circuit 2400.

Drive circuit 2400 is particularly applicable to switched mode powersupplies (SMPS). As SMPS generally require a much slower response (whencompared to the nanosecond scale used in EDM and 3D printingapplications of drive circuit 1200), the delivery of energy to a load onthe secondary side of transformer 2412 can be controlled by a sensingcircuit 2402 and control logic 2404. Sensing circuit 2402 may beconnected to the transformer primary and/or the transformer secondary.The sensing circuit 2402 may be determined to measure voltage, current,or power. An error signal detected by the sensing circuit may becalculated by determining a difference between a desired output and ameasured output. The control logic 2404 is configured to control theswitching elements 2406, 2408. The control logic 2404 may also modifythe voltage applied by voltage source 2426 in dependence on thedetermined error signal. Control logic 2404 may apply a modulationscheme whereby a digital or switched mode drive of the switchingelements or H-bridges (as described below) is controlled.

Voltage source 2426 may be controllable such that the applied voltagecan be modified as required by the particular application.

An alternative arrangement of drive circuit 2400 is shown in FIG. 24 b .Instead of the switching elements 2406, 2408, FIG. 24 b shows anarrangement in which voltage supply 2426 drives the transformer 2412 viaan H-bridge 2414. H-bridge 2414 allows the single turn primary to bealternately driven positive and negative with the single polarityvoltage supply 2426. When an H-bridge 2414 is used to control thedriving of the transformer primary, the transformer primary need nothave a CT arrangement. H-bridge 2414 may be controlled by control logic2404 (not shown). Control logic 2404 and the further circuitry connectedto the transformer secondary have been omitted from FIG. 24 b forclarity.

An alternative arrangement of drive circuit 2400 is shown in FIG. 24 c .Instead of the CT arrangement with diodes on the secondary side of thetransformer 2412 shown in FIG. 24 a , an H-bridge 2416 is provided torectify the output of the transformer secondary.

Any combination of the circuitry described as being connected to thetransformer primary can be combined with any of the circuitry describedas being connected to the transformer secondary. For example, theswitching elements 2406, 2408 connected to the transformer primary maybe provided in a circuit with the CT-diode arrangement connected to thetransformer secondary (as shown on the right of FIG. 24 a ) or theH-bridge 2416 connected to the transformer secondary (as shown on theright of FIG. 24 c ). Similarly, the H-bridge 2414 connected to thetransformer primary may be provided in a circuit with the CT-diodearrangement connected to the transformer secondary (as shown on theright of FIG. 24 a ) or the H-bridge 2416 connected to the transformersecondary (as shown on the right of FIG. 24 c ). The circuit topologiesby which the transformer 1212, 2412 may be connected to the voltagesource and the output load described herein are merely examples andfurther circuit topologies will be apparent to the skilled person.

Sensing circuit 2402 and control logic 2404 may be present in any of thecircuits shown in FIGS. 24 a, 24 b, and 24 c but have been omitted fromFIGS. 24 b and 24 c for clarity.

FIGS. 25 a, 25 b, 26 a, and 26 b show further examples ofcross-sectional views through an arm of the transformer 1212, 2412 asshown in FIG. 17 a.

FIG. 25 a shows an example in which the secondary conductors 1708passing through the inside of arm 1702 comprise a plurality of parallelplanar conductors. Planar secondary conductors 1708 are simple tomanufacture and result in greater current density and winding space uses(when compared to the non-parallel conductors shown in FIG. 17 b ). FIG.25 b shows a similar arrangement comprising two coaxial tube arms 1702.This coaxial arrangement allows the coaxial tubes to be arranged as a CTwinding. Similarly, the example arrangement shown in FIG. 17 b may beprovided with coaxial arms to allow them to be arranged as a CT winding.

FIG. 26 a shows an example in which the secondary conductors 1708passing through the inside of arm 1702 comprise a plurality of tubularconductors or solid wires. This arrangement is simple to manufacture.Using multiple smaller conductors provides a greater surface area whichreduces the skin effect and therefore affords a much lower impedance athigh frequencies when compared to a system using a single, largerconductor. FIG. 26 b shows a similar arrangement comprising two coaxialtube arms 1702.

FIG. 27 shows an example of a transformer topology for transformer 1212,2412. FIG. 25 shows an example transformer 1212, 2412 comprising twotubular arms 1702 with secondary conductors 1708 passing therethrough.Each tubular arm 1702 may be curved. As shown in FIG. 27 , each tubulararm 1702 may lie on an arc, for example the arc of a circle or anellipse. Each tubular arm 1702 may lie on the entirety of the perimeterof a circle or ellipse, or on more than 50%, 75%, or 90% of theperimeter of a circle or ellipse. As described above, the tubular armsmay be coaxial such that a centre tapped primary can be used.Transformer 1212, 2412 may further comprise interconnects 1716 a-d thatprovide connection points for the transformer to the remaining circuitry(i.e. the remainder of the circuitry shown in FIG. 12 or FIGS. 24 a-c ).One or more of interconnects 1716 a-d may comprise PCBs. Each ofinterconnects 1716 a-d may comprise PCBs. Two interconnects 1716 a, 1716d may be positioned proximal to one another, and the remaining twointerconnects 1716 b, 1716 c may be located proximal to one another andseparated from the other two interconnects 1716 a, 1716 d. Transformer1212, 2412 may further comprise a magnetic material 1718 that at leastpartially encloses the arms 1702. As described above, the magneticmaterial may comprise one or more toroidal cores. The magnetic materialmay comprise a single body with apertures through which the windings ofthe transformer extend. Preferably, the length of the transformerwindings not within the magnetic material 1718 is minimal, in order toimprove the coupling between the windings and therefore reduce theleakage inductance. The magnetic material may enclose more than 50%,75%, 80%, 85%, 90%, 95%, or 99% of the length of the windings oftransformer 1212, 2412. Insulation between arms 1702 and secondaryconductors 1708 may comprise air or a high dielectric strength material,such as ceramic, plastic, or transformer oil.

Transformer 1212, 2412 may be driven at radio frequencies (RF). Radiofrequencies are generally considered to correspond to frequencies fromaround 20 kHz to around 300 GHz. In particular, the switching elements2406, 2408 or H-bridge 2414 may be configured to allow the primary to bealternately driven positive and negative by voltage source 2426 at radiofrequencies. The transformer topology shown in FIG. 27 , having tubulararms 1702 in a circular arrangement, is particularly suited foroperation at radio frequencies. In particular, this transformer topologyprovides a high isolation and a high working voltage, and achieves thiswith exceptionally low leakage inductance. In contrast, conventional RFtransformers only achieve high isolation with correspondingly highamounts of leakage inductance. Furthermore, the low leakage inductanceexperienced by the transformer shown in FIG. 27 allows for a wideoperative bandwidth and low losses.

Resonant and quasi-resonant isolated converter circuits can be used withthe transformer 1212, 2412. The converter circuits use inductorcapacitor resonant tank circuits. It is common in LLC converters(Inductor-Inductor-Capacitor) to use the magnetising inductance of atransformer as one of the L elements and the other is implemented by theleakage inductance of a transformer. The capacitor may be an external,standard capacitor. The leakage inductance of the transformer is highlyvariable though as it depends on the precise position of the windingsrelative to each other and the transformer core. By reducing the leakageinductor to essentially zero, by using the transformer topologydescribed above in reference to FIGS. 17, 25 and 26 the use of theleakage inductance for one of the inductors (the L elements) of theresonant tank is no longer possible. However, this allows a highprecision external inductor to be used. This results in very much moreprecisely defined inductances and therefore, resonant frequency. This inturn, results in far better control of the efficiency variation indeployed systems.

Non-resonant converters switch when the voltages and currents in theconverter are non-zero. This is commonly referred to as hard switchingand results in very much higher transistor switching losses. Resonantconverters cannot be used in all applications and often hard switchingis necessary. In non-resonant converters, the efficiency may be improvedby using the transformer topology described above in reference to FIGS.17, 25 and 26 . Generally, in a conventional transformer, everyswitching cycle, as the primary is energised, the leakage inductancestores energy. When the current is reversed, the leakage inductancegenerates a flyback voltage and this must be dissipated (i.e. snubbed)by a dissipative network or by active or passive clamping. All of theseapproaches dissipate the stored energy in the leakage inductance. Usingthe transformer topologies described herein, there is close to zeroleakage inductance and therefore, the energy that needs to be dissipatedor snubbed on each switching cycle, is close to zero. Hence, thedescribed transformer topologies can greatly improve the efficiency ofall isolated switching converters.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

The invention claimed is:
 1. A drive circuit configured to drive a load,the circuit comprising: a transformer comprising: a transformer primarycomprising one or more primary windings connected to arms that passthrough a magnetic material; and a transformer secondary comprisingsecondary windings connected to secondary conductors which pass throughthe inside of the arms; wherein the drive circuit further comprises avoltage source configured to apply a voltage across the transformerprimary, and wherein a larger face of each secondary conductor is offsetfrom parallel from a larger face of the secondary conductors adjacent toit.
 2. The drive circuit of claim 1, further comprising two switchingelements configured to enable current to be passed through thetransformer primary in a first and a second direction, the seconddirection being opposite to the first direction.
 3. The drive circuit ofclaim 2, wherein if the first switching element is closed and the secondswitching element is open, a current passes through the transformerprimary in the first direction, and if the first switching element isopen and the second switching element is closed, a current passesthrough the transformer primary in the second direction.
 4. The drivecircuit of claim 1, further comprising an H-bridge, wherein the voltagesource is configured to apply a voltage across the transformer primaryvia the H-bridge.
 5. The drive circuit of claim 1, wherein the voltagesource is controllable to apply a desired voltage across the transformerprimary.
 6. The drive circuit of claim 5, further comprises sensingcircuit connected to the transformer secondary and configured tocalculate an error signal by determining a difference between a desiredoutput and a measured output.
 7. The drive circuit of claim 6, furthercomprising control logic configured to modify the voltage applied by thevoltage source in dependence on the determined error signal.
 8. Thedrive circuit of claim 1, wherein the arms comprise copper tubes.
 9. Thedrive circuit of claim 1, wherein the arms comprise coaxial tubes. 10.The drive circuit of claim 1, wherein the voltage source is connected tothe transformer primary in a centre tap arrangement.
 11. The drivecircuit of claim 1, wherein a fixed-capacitance capacitor is connectedto the transformer secondary.
 12. The drive circuit of claim 1, whereinthe secondary conductors are tubular or solid wires.
 13. The drivecircuit of claim 1, wherein the secondary conductors are planar.
 14. Thedrive circuit of claim 1, wherein the arms are curved.
 15. The drivecircuit of claim 14, wherein the arms lie on more than 50%, 75%, or 90%of the perimeter of a circle or ellipse.
 16. The drive circuit of claim1, wherein the secondary conductors are perpendicular to the side wallsof the arms.
 17. The drive circuit of claim 1, wherein the secondaryconductors are arranged in a radial formation separated by an insulatorinside the arms.
 18. The drive circuit of claim 1, wherein the secondaryconductors are equally spaced apart from each other within the arms. 19.The drive circuit of claim 1, wherein the larger faces of the secondaryconductors are offset at an angle of 60 degrees to each other.